Add helper methods to set/query the location master switch.

Consolidates the logic of exactly what the master switch means
so that if we change in the future, we don't have to find
all the different parts of the code that use it (location
settings, location quick settings, power widget, etc).

Change-Id: Ib918143b82e16541f0873c4d9141adb11fb815fc
(cherry picked from commit db3720d4)