2/ Migrate some common shell classes to use executors

- Remove Handler usage in some (non-feature specific) shell code
(Note: The shell main thread will still be the sysui main thread)
- Add explicit executor for display organizer

Bug: 161979899
Test: atest WMShellUnitTests
Change-Id: I9a354f742167b907c32537eb022e22c0d5bfb97a