Add support for sending and receiving mms when mobile data is off. Fix crash...

Add support for sending and receiving mms when mobile data is off. Fix crash during data setup error with null fail cause value.

1. Can not set up mms apn while mobile data is turned off, so it will fail
to connect when use mms connectivity in app.
Even if mobile data is turned off, we also allow set up mms data when
use mms connectivity.
2. Assign DcFailCause.UNKNOWN to fail cause if a null value is received
from DataConnection.

CRs-Fixed: 532101, 600935
Change-Id: I32eac49bec9687052ddbb5ad89da5efc478326df