Add rotation-lock to Quick Settings on phones.

Make the rotation-lock QS tile available for display on phones.

Devices < sw600dp are only allowed to lock rotation to their
natural orientation (i.e. portrait on most phones), so tweak
the QS tile label to make this clear.  e.g. "Locked to Portrait"
instead of "Rotation Locked" on portrait phones.

Simplify RotationLockController now that the sw600 check is no
longer hardcoded in RotationPolicy.

Remove redundant sw600dp check in SystemUI, everything driven
from the RotationPolicy helper, though SystemUI can still
choose not to display the tile at all with a resource config.

Clean up some of the docs in RotationPolicy to make clear the
subtle distinction between the two ways of locking rotation:
 - From Accessibility (locks to natural orientation on all devices)
 - From System UI (locks to natural < sw600dp, else current rotation)

Bug:11062710

Change-Id: I5caa4485c9501315da9fed964d6667d3012b43cb