Do less work while navigating Logins views

Fetching a set of logins from the store is quite expensive. This commit
avoids doing that while navigating back and forth between the list and
detail views:

- retain processes logins state when navigating into detail view
- use the `get` storage api to obtain specific login, instead of
  `list().filter {...}`
- avoid re-sorting retained logins when navigating back into the list
  view

app/src/main/java/org/mozilla/fenix/settings/logins/LoginsFragmentStore.kt

@@ -53,6 +53,7 @@ class LoginsFragmentStore(initialState: LoginsListState) :
 sealed class LoginsAction : Action {
     data class FilterLogins(val newText: String?) : LoginsAction()
     data class UpdateLoginsList(val list: List<SavedLogin>) : LoginsAction()
+    object LoginsListUpToDate : LoginsAction()
     data class UpdateCurrentLogin(val item: SavedLogin) : LoginsAction()
     data class SortLogins(val sortingStrategy: SortingStrategy) : LoginsAction()
     data class DuplicateLogin(val dupe: SavedLogin?) : LoginsAction()
@@ -97,6 +98,9 @@ private fun savedLoginsStateReducer(
     action: LoginsAction
 ): LoginsListState {
     return when (action) {
+        is LoginsAction.LoginsListUpToDate -> {
+            state.copy(isLoading = false)
+        }
         is LoginsAction.UpdateLoginsList -> {
             state.copy(
                 isLoading = false,
@@ -125,9 +129,7 @@ private fun savedLoginsStateReducer(
         }
         is LoginsAction.LoginSelected -> {
             state.copy(
-                isLoading = true,
-                loginList = emptyList(),
-                filteredItems = emptyList()
+                isLoading = true
             )
         }
         is LoginsAction.DuplicateLogin -> {

app/src/main/java/org/mozilla/fenix/settings/logins/controller/SavedLoginsStorageController.kt

@@ -203,35 +203,31 @@ open class SavedLoginsStorageController(
         loginsFragmentStore.dispatch(LoginsAction.DuplicateLogin(dupe))
     }
 
-    fun fetchLoginDetails(loginId: String) {
-        var deferredLogin: Deferred<List<Login>>? = null
-        val fetchLoginJob = lifecycleScope.launch(ioDispatcher) {
-            deferredLogin = async {
-                passwordsStorage.list()
-            }
-            val fetchedLoginList = deferredLogin?.await()
-
-            fetchedLoginList?.let {
-                withContext(Dispatchers.Main) {
-                    val login = fetchedLoginList.filter {
-                        it.guid == loginId
-                    }.first()
-                    loginsFragmentStore.dispatch(
-                        LoginsAction.UpdateCurrentLogin(
-                            login.mapToSavedLogin()
-                        )
+    fun fetchLoginDetails(loginId: String) = lifecycleScope.launch(ioDispatcher) {
+        val fetchedLogin = passwordsStorage.get(loginId)
+        withContext(Dispatchers.Main) {
+            if (fetchedLogin != null) {
+                loginsFragmentStore.dispatch(
+                    LoginsAction.UpdateCurrentLogin(
+                        fetchedLogin.mapToSavedLogin()
                     )
-                }
-            }
-        }
-        fetchLoginJob.invokeOnCompletion {
-            if (it is CancellationException) {
-                deferredLogin?.cancel()
+                )
+            } else {
+                navController.popBackStack()
             }
         }
     }
 
     fun handleLoadAndMapLogins() {
+        // Don't touch the store if we already have the logins loaded.
+        // This has a slight downside of possibly being out of date with the storage if, say, Sync
+        // ran in the meantime, but that's fairly unlikely and the speedy UI is worth it.
+        if (loginsFragmentStore.state.loginList.isNotEmpty()) {
+            lifecycleScope.launch(Dispatchers.Main) {
+                loginsFragmentStore.dispatch(LoginsAction.LoginsListUpToDate)
+            }
+            return
+        }
         var deferredLogins: Deferred<List<Login>>? = null
         val fetchLoginsJob = lifecycleScope.launch(ioDispatcher) {
             deferredLogins = async {

app/src/test/java/org/mozilla/fenix/settings/logins/LoginsFragmentStoreTest.kt

@@ -167,7 +167,7 @@ class LoginsFragmentStoreTest {
         store.dispatch(LoginsAction.LoginSelected(mockk())).joinBlocking()
 
         assertTrue(store.state.isLoading)
-        assertTrue(store.state.loginList.isEmpty())
-        assertTrue(store.state.filteredItems.isEmpty())
+        assertTrue(store.state.loginList.isNotEmpty())
+        assertTrue(store.state.filteredItems.isNotEmpty())
     }
 }

app/src/test/java/org/mozilla/fenix/settings/logins/SavedLoginsStorageControllerTest.kt

@@ -92,14 +92,14 @@ class SavedLoginsStorageControllerTest {
             httpRealm = "httpRealm",
             formActionOrigin = ""
         )
-        coEvery { passwordsStorage.list() } returns listOf(login)
+        coEvery { passwordsStorage.get("id") } returns login
 
         controller.fetchLoginDetails(login.guid)
 
         val expectedLogin = login.mapToSavedLogin()
 
         coVerify {
-            passwordsStorage.list()
+            passwordsStorage.get("id")
             loginsFragmentStore.dispatch(
                 LoginsAction.UpdateCurrentLogin(
                     expectedLogin