Annotate setUidFirewallRule with @GuardedBy

This method is only called with the lock held.
Annotating it helps ErrorProne verify calls to other methods that
require the lock to be held are okay.

Test: Build
Bug: 220270435
Change-Id: I1f6c3c9e339935c06b3edd5436b2b1a7922a67ed
1 file changed