android: Override `app_name` string for debug build.
android: Override `app_name` string for debug build.

This causes a resource reference like `@string/app_name` -- which we
use for `android:label` on our application in the manifest -- to have
the new value "Zulip (debug)" in the debug build, while it remains
"Zulip" in a release build.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile