Ensure cleanup when interrupting scroll capture

A batch of related cleanups and fixes:

 * Connects cancellation signals between the layers of scroll capture.

 * Handles cancellation on dismissal, ensuring clean shutdown of session.

 * Ensures any pending operation is cancelled on the client side and the
   connection is closed.

 * App side of connection handles a crash of SystemUI, ensure close()

 * ScrollCaptureController:
   Moves onCaptureResult to a background thread; it calls into binder.

 * Fixes accuracy and category of some log messages about dismissal
   gesture.

Test: atest ScrollCaptureConnectionTest ScrollCaptureControllerTest
Change-Id: I20014a4cb43e28fcb97f6980fb47af8c06deaca2