Hacky workaround for half-finalized builds.

Metalava increments the SDK level by one when it's not "REL", so we
temporarily force the build to be "REL" while we're still in the
process of finalizing it.

This CL must be reverted as part of actually declaring "REL".

Bug: 171506470
Test: Build
(cherry picked from commit 05597c0c)

Change-Id: I18405cdd27769598f5e7a6a968f0dd473f936f6c