Keep only one back animation playing.

- Preventing from prepare a new animation while previous one is
playing, also clear up animation resources if gesture was finished
before onAnimationStart.
- Extends the monitor periods until transition finish.
- There do not really need to hide the close target after animation
finish, transition system will do it.
- Reset launchTaskBehind flag after transition finish, there won't
collect the animating target if there is no visibility change.

Bug: 131727607
Bug: 208786853
Bug: 274997465
Test: atest BackNavigationControllerTests
Test: monkey test and I'm the monkey, randomly start activity and
trigger back ASAP.
Test: trigger the back-to-task animation several times, and verify that
the flickering no longer occurs.
Test: open app, and trigger a back-to-home animation, verify the
launchTaskBehind flag of the home activity will be reset after transition
finish.

Change-Id: I736b987370c06b24cb5285e828fa482843b0cfb8