I've finally managed to build the release of KOreader for Android, but I've run into some issues.
I've updated the manual to reflect the solutions I've found.
The issues I've found are:
1. The version of the needed Android SDK platform isn't described anywhere;
2. xutils-dev is missing from the prerequisites, while it provides the `makedepend` utility used by the openssl build system;
3. Finally, for the Android build it is necessary to install JDK version 8.