Prevent NPE in ActivityStack#shouldbeVisible.

If there is a visibleBehind activity, we check to see if the top
activity in the top stack is not fullscreen. However, it is possible
for the top stack to be empty. This can happen if the previously top
activity is being re-installed.

This changelist addresses the issue by checking if the top activity is
null before referencing it.

Change-Id: I6de904bed1c7035ed1e112c9cacc1b6c2bac4e8f
Fixes: 32180256
Test: bit FrameworksServicesTests:com.android.server.am.ActivityStackTests#testShouldBeVisibleWithVisibleBehindActivity