Fix NSSL flicker when going to/from OCCLUDED

There was a small flicker as the alpha would be set to 1 briefly
during the transitions. It turns out the legacy shade interactor may
send an event indicating the shade is fully expanded when it
resets. This is followed by an event that says it's now collapsed but
the damage is done. Prevent this by never interrupting the transitions
from KeyguardTransitionRepository.

Fixes: 314290249
Test: Manual - observe the one frame flicker no longer happens during
OCCLUDED<->LOCKSCREEN
Flag: ACONFIG com.android.systemui.keyguard_shade_migration_nssl
DEVELOPMENT

Change-Id: I460dfc11a55d94d07db9a94b00c58b9fdfda2979