Delay onBackStarted() until the pointer is pilfered.

This only delays the dispatching of the start event. We still start back navigation as early as the first MOVE, to allow more time to receive back navigation info and avoid jank.

Bug: 308550613
Bug: 301195601
Test: atest OnBackInvokedCallbackGestureTest
Test: atest BackAnimationControllerTest
Flag: NONE

Change-Id: I0802fd716dc5dbce7071e083bd373df5c5ddcc4c