Bug 1194721: Support Gonk sensors daemon, r=gsvelto

This patch adds th state machine for supporting the Gonk sensors
daemon in Gecko. The daemon gets started when the first sensor is
enabled. Sensors can be enabled and disabled at will. The daemon
will send events about detected sensors and sensor events. Gecko's
state machine receives themand forwards them as DOM events.

The old support for device sensors is still present for devices without
sensorsd. A future patch will remove this code.