1/ Initial preparation for separate shell main thread

- Add @BinderThread/@ShellMainThread/@SysUIMainThread annotations
  for some common shell classes and callbacks
- Add mechanism to inject shell-anim/shell-main/sysui-main/etc
  executors (removed AnimationThread static instance)
(Note: The shell main thread will still be the sysui main thread)

Bug: 161979899
Test: atest WMShellUnitTests

Change-Id: I4adfeaebe5b79fab7c9451eef7b54fcdf4d8d40a