android build: Bump Support Library version to 28.0.0.
android build: Bump Support Library version to 28.0.0.

Follows facebook/react-native@5bbed4385 upstream, one of the changes
in RN v0.59.

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