Delay keyguard hiding until after occlusion animation ends.

The keyguard should remain visible during the occlusion animation
since the occluding activity is launching over it. The hideKeyguard
call is also extremely janky and causes the animation to be nearly
imperceptible through the jank.

See https://b.corp.google.com/issues/250542279#comment15 for more
context.

Fixes: 250542279
Test: atest KeyguardViewMediatorTest
Test: atest CentralSurfacesImplTest
Test: launch camera, notice far less flickering
Change-Id: Ic3f7e62a42829481f6809d32ee1aa4690927ece8