Introduce transparent bars to the keyguard.

Driven by a new framework config bool. Tested on all layouts,
but disabled by default.  Can be enabled with a config overlay.

(Sliding/MultiPane)ChallengeLayout have custom measure/layout
logic that peeks at the root view's padding (!).  So we must keep
the root view's padding = system window insets. However, we need
the scrims + the fullscreen camera widget preview to use the entire
display size if the bars are transparent.

So the approach is to maintain the root view's
(KeyguardViewManager$ViewManagerHost) padding as before via
setFitsSystemWindows(true) but allow non KeyguardHostViews to
extend into the padding if bars are transparent via a custom
measure/draw on the root view.

KeyguardHostView background + challege scrims moved up to the
root view.

Also:
- Remove logspam line for fixed bug.

- Fix status bar staying opaque on interaction on secure keyguard.

Known issues:
- Nav bar blip dismissing global actions, keyguard app widget picker
bars are opaque.  Both will use the new inherit flag once it exists.

- Emergency dialer layout dance, will be fixed in the phone project.

Bug:10606085
Change-Id: I2648b4ee9a54a67324ce9dcbab976ad4da4bbcfa