Enable content capture running on background thread

1. Make sure works inside MainContentCaptureSession are delegated to the
   assigned thread. So even when the works are not done in UiThread, the
   synchronization is still guaranteed.
2. Remove requirement of ContentProtectionEventProcessor running on
   UiThread. MainContentCaptureSession is the only caller to the
   processor. And it is guaranteed that only the assigned thread will be
   used. So no need to restrict it on UiThread.
3. Move notifyContentCaptureEvents work away from ViewRootImpl and let
   MainContentCaptureSession decide how to process this.

Test: CtsContentCaptureServiceTestCases, MainContentCaptureSessionTest.
BUG: 309411951
Change-Id: I13ce958da2280171fb2eb85b3321e3c7a7c5d0aa