Use a config to enable MMS when mobile data is off

Some operators require that an MMS be allowed even when data
is turned off. Introduce a config to determine if MMS should
be allowed. By default this config is set to false.

Change-Id: Ic90dbac6084e62338cb46d1caf881a0ecc4b674b

Cleanup connections for APNs that are not connected

- After disabling a network, ConnectivityService tears down
  the data connection
- However, for APNs that are in Connecting, Failed or Retry
  state, ConnectivityService will not do that because the NetworkAgent
  is created only when a DataConnection goes into Active State.
  Take care of this case by explicitly tearing down the data connection

Change-Id: I87f286cb8b5fe5c6a33e8c5cf53da7bca8d7aa2b

DCT: Tear down MMS+default connections raised by config_enable_mms_with_mobile_data_off

If the tracker lets an MMS connection go through even if data is off, make sure
it's torn down once the disabled state gets applied. More often than not, APNs
configured as MMS+data would remain alive, and the tracker's state machine would
get stuck out of sync

Change-Id: Ia13e9ff5beea44ecfda40a4910990dab53af25dd