Only execute relayout after animation is over

After the keyguard dismissal animation is over, there will be an
expensive frame where a relayout is needed.

This relayout should only happen after processing the whole animation,
otherwise there will be jank. In order to do so, we'll post
onKeyguardExitFinished() and related methods to the next frame.

Test: atest KeyguardViewMediator
Test: perfetto trace
Fixes: 210434695
Change-Id: Ic2387474e095189ed2e23b177c243247af2198bf