Update reference to top activity.

A condition dealing with a paused top activity references the last
paused activity rather than the top activity. While technically
correct, this may be confusing. Updating to solely reference the
top activity.

Fixes: 65585235
Test: cts/hostsidetests/services/activityandwindowmanager/util/run-test CtsServicesHostTestCases android.server.cts.ActivityManagerDisplayTests#testExternalDisplayActivityTurnPrimaryOff
Change-Id: Ie67cb8b103e7dec59b1c0b40a5bb35ef0c3cb2b7