Add developer option control for GNSS duty cycling.

Duty Cycling control is one of the Android-P features. In Android-P the
feature will be controlled via a Developer Options button with no
public API. This CL adds the button and connect it to be read via the
GnssLocationProivder to control the feature. Also Roboelectric test
added.

Bug: 64009176
Test: Tested on Android-P build and works as expected. Also Robolectric test added.
Change-Id: Icc0d259719cf9034caf2c78c790b914ab4292597