Reduce activity lifecycle churn when exiting split-screen mode.

- Ignore calls to resize the docked stack due to it detaching while we are
already resizing stacks due to the docked stack switching to fullscreen mode.
- Defer resuming activities in a stack we are resizing to fullscreen when the
docked stack is going away. The activities may not be the top most activity
anymore since the activity in the docked stack might be moved on top of it.
- Removed call to ensure configuration in ASS.positionTaskInStackLocked.
This isn't needed as the ensure activities visible call will make sure
the activity is in the right configuration if it is going to be visible and
we don't want to be relaunching it if it isn't going to be visible.

Bug: 28196862
Change-Id: I26460dd9c649b8ec1bf296d5182e9ff6cafe6df0