Expose windowBlurRadius as a flow that is in sync with various bouncer transitions

All transitions to and away from the bouncer will provide an implementation of how the windowBlurRadius should change.
This will be used by WindowRootViewModel to apply the specified blur to the window root view.

Bug: 370555003
Flag: com.android.systemui.bouncer_ui_revamp
Test: Added unit tests
Change-Id: I002141d69a41b44099e434e38b4c80935b2b44a2