`android_release_checker`: fix strings check on release branch

`android_release_checker`'s [check_modified_strings_on_release](https://github.com/Automattic/dangermattic/blob/df75d47e76b3ae465e5b5ac41ce970b5e39645d6/lib/dangermattic/plugins/android_release_checker.rb#L32) should skip base strings, or, better yet, we should add a `check_modified_translations_on_release_branch` similar to [the one we have on iOS](https://github.com/Automattic/dangermattic/blob/df75d47e76b3ae465e5b5ac41ce970b5e39645d6/lib/dangermattic/plugins/ios_release_checker.rb#L75C9-L75C54).

Start in lib/dangermattic/plugins/android_release_checker.rb at check_modified_strings_on_release, then compare the analogous check_modified_translations_on_release_branch in lib/dangermattic/plugins/ios_release_checker.rb. Determine whether Android should skip base strings or mirror the iOS translation check; done means the release-branch check no longer flags base strings incorrectly.

Written by the indexing model from the issue text.