Keep built APKs and test screenshots out of the repo

Two 24MB rollback APKs and a screenshot reached a commit through a
broad `git add`. The rollback series is archived on the box at
/srv/prod/messenger/apk-rollback, which now holds the full run from 0.6
to 0.25.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitignore

@@ -2,3 +2,11 @@ build/
 .gradle/
 .DS_Store
 local.properties
+
+# Built and rollback APKs, and screenshots taken while testing. The rollback
+# series is archived on the box at /srv/prod/messenger/apk-rollback, which is
+# where it belongs: a 24MB binary per release makes every clone of a public
+# repo carry it for ever.
+*.apk
+apk-rollback/
+Screenshot_*.png