Clear un-sent pending animation if back gesture is complete finished.

Shell could finish back gesture before pending animation start. Clear
the original pending animation in case it being sent accidentally.
Also setLaunchBehind doesn't need to call setVisibility, since there
already calls commitVisibility. This also the reason where perform
surface placement unexpected occur during prepare animation.

Bug: 319293406
Bug: 319293969
Test: simulate calls onBackNavigationDone before pending animation
start, verify previous pending animation won't be triggerred while
start second back navigation again.

Change-Id: I28867f91b0095b23c377d5fbd2e2fb2add39f9c3