Make ContentCapture calls directly from the UI thread.

Most calls are bufffered anyways, so by not using a handler we save time spent
on getting a pooled lambda and posting to the the handler.

The only "expensive" operation is flushing, but it makes a oneway binder call
so it might be fine (and if necessary, we can optimize it later).

Bug: 123307965
Test: atest CtsContentCaptureServiceTestCases
Change-Id: I7182c8e193f58fa000396fdb3003e771214bf79b