Remove custom Handler from StatusBar.

Replaces the custom "H" subclass from StatusBar and replaces
it with MessageRouter.

Remove a few spurious messages that would be passed to it, but never
processed. These are MSG_PRELOAD_RECENT_APPS,
MSG_CANCEL_PRELOAD_RECENT_APPS, and MSG_HIDE_RECENT_APPS.

Also, replace various mHandler#post calls with mMainExecutor#execute.
Virtually all usages of Handler inside of StatusBar are now
elimanted.

Bug: 190746471
Test: atest SystemUITests
Change-Id: I6a3b2b226108ef3abed96a1007978491ec50fcb0