Use sensorfw for ambient light and proximity sensor input

Tracks sensord D-Bus name on system bus to detect availability
of the sensors API.

While the D-Bus name is not reserved, ALS and PS enable states
are cached internally within MCE. When sensord startup / restart
is detected the cached state is used for opening sensor sessions
as needed.

[mce] Use sensorfw for ambient light and proximity sensor input
Merge pull request #69 from spiiroin/use_sensorfw

Use sensorfw for ambient light and proximity sensor input