Have MobileDataStateTracker & DataConnectionTracker communicate directly.

Added CMD_SET_DATA_ENABLE which is sent when data is enabled/disabled
via the ConnectivityService. It is anticipated that the communication
channel will be used for additional commands and to receive unsoliciated
commands from DataConnectionTracker back to MobileDataStateTracker.

Change-Id: I3863e7385155d503f069b1dcb7e4f766ec78b5f8