Close #15979: Add MOZILLA_OFFICIAL flag for release builds

When we build release APKs on Mozilla infrastructure, we want a way to
know this in code for features that would only work on them.

app/build.gradle

@@ -360,6 +360,16 @@ android.applicationVariants.all { variant ->
         buildConfigField 'String', 'NIMBUS_ENDPOINT', 'null'
         println("--")
     }
+
+// -------------------------------------------------------------------------------------------------
+// BuildConfig: Set flag for official builds; similar to MOZILLA_OFFICIAL in mozilla-central.
+// -------------------------------------------------------------------------------------------------
+
+    if (project.hasProperty("official") || gradle.hasProperty("localProperties.official")) {
+        buildConfigField 'Boolean', 'MOZILLA_OFFICIAL', 'true'
+    } else {
+        buildConfigField 'Boolean', 'MOZILLA_OFFICIAL', 'false'
+    }
 }
 
 androidExtensions {

taskcluster/fenix_taskgraph/transforms/build.py

@@ -104,7 +104,10 @@ def add_nightly_version(config, tasks):
 
     for task in tasks:
         if task.pop("include-nightly-version", False):
-            task["run"]["gradlew"].append('-PversionName={}'.format(formated_date_time))
+            task["run"]["gradlew"].extend([
+                '-PversionName={}'.format(formated_date_time),
+                '-Pofficial'
+            ])
         yield task
 
 
@@ -112,9 +115,10 @@ def add_nightly_version(config, tasks):
 def add_release_version(config, tasks):
     for task in tasks:
         if task.pop("include-release-version", False):
-            task["run"]["gradlew"].append(
-                '-PversionName={}'.format(config.params["version"])
-            )
+            task["run"]["gradlew"].extend([
+                '-PversionName={}'.format(config.params["version"]),
+                '-Pofficial'
+            ])
         yield task