Fixes app crash when starts activity with FLAG_ACTIVITY_REORDER_TO_FRONT

Application was crashed while starting an embedded Activity with
FLAG_ACTIVITY_REORDER_TO_FRONT, because the embedded Activity was
not the direct child of the Task.

In this CL, the embedded activity is now moved to the top-most
position of the Task and dismissed from being embedded in order
to honer FLAG_ACTIVITY_REORDER_TO_FRONT.

Bug: 255701223
Test: locally verified with app
Test: atest TaskTests
Change-Id: I491139e5e1d712993f1fef9aebbd75e9ccfc539e