Transitions - Allow ->GONE to be canceled

Especially for when the user may auth but continue pushing down the
power button, allow transitions to GONE to be interrupted by an
immediate transition back to any keyguard state.

Also, optimization for DREAMING->LOCKSCREEN. Add a slight delay in
processing events, because the data source events may come in out of
order or with a small gap.

Test: existing transition tests cover these small tweaks well
Test: atest
frameworks/base/packages/SystemUI/tests/src/com/android/systemui/keyguard/
Fixes: 265325414

Change-Id: I07d8b225db2df04453ffe4b694f5124da94ee560