Fix pressing back on bouncer

If the user goes back when on Bouncer, do not animate the panel up. In
particular, DO NOT just animate the header out.

Additionally, only reset the bouncer if it's scrimmed and not
needFullScreen.

Test: manual, use flow of b/130525889
Test: swipe up in Keyguard and press back goes back to keyguard
Test: Expand shade and press Reply on notification, going back on
bouncer shows expanded shade
Test: Open camera, open gallery, going back on bouncer goes back to
camera
Test: SIM card bouncer, going back does not break
Fixes: 130525889
Change-Id: I122cf0858acbfdaa514efb32190369d95df6e001