Let keyguard (un)occlude animation run even when no activity exists.

Keyguard (un)occlude transition can be executed even before the
occluding app becomes ready. Normally we skip to run remote animation
for such cases since nothing to animate, however, KeyguardService uses
IRemoteAnimationRunner#onAnimationStart as a signal to update its
internal state, so WindowManager has to trigger the binder call.

Bug: 175687166
Bug: 182975035
Test: atest AppTransitionControllerTest
Change-Id: I190f2cf750f849b8aafdb43642cb03e5382f50d5