Moves focus to the adjacent embedded activity on back nav

In order to provide a consistent back behavior in AE, the
system now finish the most recently active activity when
back pressed, regardless whether the activity is the top-most
activity on the Task.

This is done by comparing the creation time of the Activity.
Or the creation time of the additional app window if any.

Also removes TF#mDimmerSurfaceBoosted since it is no longer
needed.

Bug: 240575809
Test: atest TaskFragmentTest
Test: atest BackNavigationControllerTests

Change-Id: Iad1c8a92f12596dc83e3cc8d7aa044c507f07733