Rotation lock.

IWindowManager now supports two new methods,
freezeRotation() and thawRotation(), that allow a caller to
temporarily stash the device's current rotation as the
default rotation (when no other constraints are present).

The system bar uses this to implement a user-accessible
rotation lock by calling freezeRotation() and then turning
off accelerometer-based display rotation; unless overridden
by an app, the display will continue to appear in the frozen
rotation until the rotation is unlocked by the user (either
via the rotation lock icon in the system bar or by checking
"rotate screen automatically" in Settings).

Bug: 2949639
Change-Id: Icd21c169d1053719590e72401f229424b254622f