Ensure all stacks got examined for removal

The top activity of the topmost stack was resumed. The next
focusable activity/stack would be brought to top while
removing the topmost stack. Since the stack order changed,
we did not remove the newly topmost stack while continuing
to remove stack one-down.

Bug: 121105810
Test: atest ActivityDisplayTests
Test: atest ActivityManagerDisplayLockedKeyguardTests

Change-Id: I4f840b8aa87c2bc3563572d83e9cd424e7a3e023