Distinguish transient + transparent bar styles.

Transient bars = 30% black scrim per spec (formerly view alpha).
Transparent nav bar = Simple gradient drawable from 50% to 0% black.
Transparent status bar = Fully transparent.

Smoother transitions between bar modes will be tricky due to
coordination with window manager, and thus deferred to a separate
future change.

Pre-baked assets instead of the gradient drawable will also be
part of a separate future change, this change is simply meant to
move closer to the final look and feel to minimize confusion for
those kicking the tires on the new flags.

Bug:10012887
Change-Id: I8c35a660187903224a6bfe604b5313e9115c4a73