Add flag for back button on bouncer

For desktop-form factor, we want to add a 'back' button to the bouncer UI. This flag will guard this behavior.

Test: manual - adb shell aflags list
Flag: com.android.systemui.back_button_on_bouncer
Bug: 392597328
Fixes: 409001131
Change-Id: Ie4bb22f66783bc130ebe7e2aeaa57138aa340dbf