Implement multi-press behavior for power key.

Added support for brightness boost or setting theater
mode from the power key.  This behavior is disabled by default and
must be enabled for specific products in a config.xml overlay.

Because the power key is already so overloaded, refactored the
code to split out handling of the similar ENDCALL button and
renamed all of the state that has to do with screenshot chord
detection to avoid confusion.

Bug: 17949215
Change-Id: Id282133188e3781472aabb64fabcee7b98d0c77d