Prepare ViewStructures on UiThread

1. onProvideContentCaptureStructure should always be called from the
   UiThread to prevent concurrrent issue. Therefore,
   notifyContentCaptureEvents needs to prepare ViewStructures on the
   UiThread first before sending event out.
2. Fork MainContentCaptureSessionV2 to hold the logic of running parts
   of the operations in the background thread. And let
   ContentCaptureManager assign different implementation dependes on the
   flag. This should reduce the potential impact on the control arm.

Test: CtsContentCaptureServiceTestCases, MainContentCaptureSessionTest.
BUG: 309411951

Change-Id: I6efee6e94e2b9857b2ad93036aa28dcd5d3a263c