Simplify condition to check resumed activity when unlocking

The latest resumed activity will update to mTopApp. So it is enough
to check from the global top rather than looping each display. Then
it will be simpler to avoid calling keyguardGoingAway for each display
in the future.

Bug: 423613749
Flag: EXEMPT bugfix
Test: atest KeyguardLockedTests
Change-Id: I634ebd29e513502a0ded80f6e3391a3bd59ab423