Provide an Android version code derived from the Emacs version

Provide an Android version code derived from the Emacs version

The version code is intended to be an integer that increments
for each Android package release
(https://developer.android.com/studio/publish/versioning#versioningsettings).

If we keep this updated under version control, then F-Droid (a
third-party Android package repository), can watch for that, and
use it to automatically build Emacs packages for Android each
time a new Emacs release is tagged
(https://f-droid.org/en/docs/Build_Metadata_Reference/#UpdateCheckData).

* admin/admin.el (set-version): Update version code in
java/incrementing-version-code
* java/incrementing-version-code: New file containing an Android
version code corresponding to the current Emacs version.
(bug#75809)