Disallow pinned stack to control occluded state.

In the original design, we check the top focus stack of each display
to control its occlusion state. However, the start time of pinned activity
may start later than the user's application, such as PipMenuActivity. In
which case the top focus activity will not be what the user expects.
To prevent this, we can ignore Pinned stack when checking visibility for
a display.

Fix: 120445909
Test: atest KeyguardTests KeyguardLockedTests
Test: atest ActivityManagerMultiDisplayTests
Test: atest ActivityManagerDisplayLockedKeyguardTests ActivityManagerDisplayKeyguardTests

Change-Id: I3be19a803aa59c21841e476aa46a0222d156e21e