The big Keyguard transition refactor (8/n)

Don't force mKeyguardGoingAway, as this never recovers. Make sure
to only show the dismissing Keyguard activtiy and recover the
state when trusted state changes.

Test: Make sure Keyguard is in a trusted state, start an activity
with FLAG_DISMISS_KEYGUARD from FLAG_SHOW_WHEN_LOCKED activity
and make sure there is no flicker.

Bug: 32057734
Change-Id: I5d212f6f9d5430250b22c8370f45dc95756432d2