android build: Update to Android Gradle Plugin v3.3.1.
android build: Update to Android Gradle Plugin v3.3.1.

Release notes, in full (vs. 3.3.0 which we were already on):

  This minor update supports Android Studio 3.3.1 and includes various
  bug fixes and performance improvements.

Sure, OK.

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