android build: Upgrade Android Gradle Plugin to v3.5.0.
android build: Upgrade Android Gradle Plugin to v3.5.0.

Release notes here:
  https://developer.android.com/studio/releases/gradle-plugin#3-5-0
Said to improve build speed, which sounds nice.  No major
potentially-incompatible changes identified there.

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