Use shared flow for keyguard going away

It is possible for a slow consumer to receive conflated values from
a state flow. Events like keyguardGoingAway should never be conflated,
or visual glitches could occur.

Fixes: 407825005
Test: atest KeyguardRepositoryImplTest
Flag: EXEMPT bugfix
Change-Id: I23d72503c2b44e78dcfd5533f2f1a34d8c7bdf6e