Use shared flow for transition animations

There was state incorrectly being shared between multiple
flows. Convert to a SharedFlow instead, to more efficiently reuse.

Also add optional animation logging in a new logbuffer.

Also convert all transition tests to use Kosmos.

Bug: 296373465
Test: atest com.android.keyguard.systemui.keyguard.ui.viewmodel
Flag: N/A
Change-Id: Ic0f507ad90b5c8a4643ab31cd24aa65a6686ecc0