android: Bump compileSdkVersion to 27.
android: Bump compileSdkVersion to 27.

This version number doesn't have much effect, except that it must be
at least targetSdkVersion.  So this upgrade is easy.

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