Do not force-exit desktop immersive during recents transition

Avoids forcing a task out of desktop immersive during the recents
transition by tracking the start/finish of the transition.

Also adds exit reasons to desktop immersive exit points, for more
informative logs.

Fix: 377716657
Flag: com.android.window.flags.enable_fully_immersive_in_desktop
Test: Open Netflix in desktop, play video, enter desktop immersive,
swipe up to Overview. Verify Netflix is not forced out of desktop
immersive.

Change-Id: I8f68786c558131cbd56e8404dac8c03d7346b660