Android.gitignore: Ignore *.aab

[Khazbs]

Reasons for making this change:

"After August 2021, all new apps and games will be required to publish with the Android App Bundle format." Generated Android App Bundles (*.aab), just like Android Packages (*.apk), should not be checked into VCS.

Links to documentation supporting these rule changes:

Android Developers Blog: New Android App Bundle and target API level requirements in 2021