[flexiglass] Moves alternate bouncer (back) to its own window.

Fix: 379847843
Test: manually verified that hitting a notification while locked shows
the UDFPS alternate bouncer, that using the right/wrong fingerprint
works as expected to unlock/bring up the bouncer, that canceling out
from the bouncer and repeating, correctly does it again.
Flag: com.android.systemui.scene_container

Change-Id: I17877b823b4e2ae638b701a1fc3d686fda0b8f9a