Specify target_sdk_version to be 30.

This is a revert to ag/11526308, so we get a stable platform release and
don't have to deal with P0 test failures as platform features get
developed.

Bug: 178054993
Test: atest DocumentsUIGoogleTests
Change-Id: I64bc2fe98b6278324412ef5b99cbe2dbd7a73745