Start blocking relayout sooner in exit/cancel enter desktop transitions.

Resolves a flicker issue in exit desktop and cancel enter desktop
transitions caused by relayout occurring before the animations
start. Relayout is now blocked in exit desktop transition and sooner for
cancel enter desktop transition.

Bug: 280834154
Test: Manual
Change-Id: I83c92d23b5d860bc558263cdb2acff281999026d