Tweak shell thread and clean up some code

- Make shell main thread display priority to match boosted code in
  wm core
- Make shell thread handlers async
- Add tracing and logs for debug builds

Bug: 181979527
Test: atest WMShellUnitTests
Change-Id: Ib3494d77317b24897f5f3738814b5977cd5a2d40
2 files changed