Ensure keyguard alpha is 0f on OCCLUDED->PRIMARY_BOUNCER

When apps such as home controls launch from an expanded shade,
the shade collapsing may reset alpha to 1f. Reassert the alpha
to be 0f.

Fixes: 397867199
Test: atest OccludedToPrimaryBouncerTransitionViewModelTest
Flag: EXEMPT bugfix
Change-Id: I82f03ff0feac5aae622adb77a38f003fba0fa808