[flexiglass] Removes BackHandler from AlternateBouncer

Double checked on pre-flexiglass. Back navigation isn't possible in
pre-flexiglass when the alternate bouncer is shown.

Fix: 360109685
Test: manually verified that tapping on the alternate bouncer empty
space still opens the primary bouncer and that backing out of _that_
correctly puts me back on the occlusing Photos activity (following the
reproduction steps of the attached bug)
Test: manually verified no harm done: using the correct finger in
alternate bouncer correctly unlocks the photos app
Flag: com.android.systemui.scene_container

Change-Id: Ie1fb59da36beafb4d0649e245c423911f53bdea5