[flexiglass] Lockscreen scene

Adds the standalone KeyguardRootView to LockscreenScene.

Note how we remove it from its parent ViewGroup, if there is one,
whenever our view factory gets called. This is needed so there is only
one KeyguardRootView at all times (or it crashes because the NSSL gets
initialized twice).

Bug: 280879610
Test: manually verified that the lockscreen scene renders the real
lockscreen
Test: manually verified that pulling down brings down the shade and
pulling up brings up the bouncer
Test: manually verified the ability to unlock via PIN bouncer and face
unlock and then re-lock the device via power button + timeout - no crash
Test: manually verified correct rendering in AOD

Change-Id: I54c11bccc7a10e43afea5dbaa09013e72b0a806f