Use main executor to run remote animation from notification

The pending intent of notification may launch the activity indirectly,
e.g. service or receiver. Before the activity is started, the
notification may be canceled, then the notification view is detached.
And View#post will only put the runnable to a list that will be
executed until attach again. So in this case since the notification
view is removed, the remote animation is never executed that leads to
remote animation timeout and the activity jumps cut without animation.

This fix ensures that the handler to animate is valid.

Also fix a flickering issue that if activity doesn't draw fast enough
(ActivityLaunchAnimator#ANIMATION_DURATION) then the notification panel
has started to collapse. The notification view may have clip bottom
larger than its actual height, that causes negative crop (as no crop)
height of the animating surface. So the first frame will show fully
height and then start to animate from the height close to zero.

This fix ensures that the crop height is at least zero.

Fixes: 158207087
Test: atest ActivityLaunchAnimatorTest
Test: Send a notification which will start a service. The service
      cancels the notification and starts a activity with >500ms
      delay. Check the launch animation should run smoothly.

Change-Id: I78426db1ce295c633271fcaa685b47cc9f697761