Add TF flag to allow transitions when empty

By default, the TaskFragment waits until it becomes non-empty to start
transitions. If mAllowTransitionWhenEmpty is set to true, then it will
start transition even if it is empty. This is need for Launcher to
create an empty TF and obtain the surface before launching the -1
activity.

Bug: 320720349
Test: atest TaskFragmentTest
Change-Id: Ie7aca6391032e660dd2bc243ddba53db945af09f