Notify modem about screen on/off states more precisely.

The SCREEN_ON / SCREEN_OFF broadcasts are no longer adequate for
monitoring the screen state since they are not sent in cases
where the screen is turned off transiently such as due to the
proximity sensor.

This change uses the DisplayManager to watch for screen state
changes to tell the RIL that it can stop sending signal strength
updates.

It's worth pointing out that the RIL should *not* be watching for
screen state changes in the first place!  We should create a
new API to more precisely specify the behavior to be controlled
and the signals which govern that behavior.  But this change
should be good enough to save a little more power today when the
screen is off due to the proximity sensor.

Bug: 17821361
Change-Id: Ide77e04c3b2cadefc44ef5e51bd958f25a4a3d62