Ensure only MMS is allowed when data disabled but MMS allowed

If data is disabled but MMS is allowed unconditionally via user
settings, mark the data connection for MMS use only. This prevents
internet from being added onto a connection that's intended for MMS use.

Test: manually verify before/after
Test: atest DcTrackerTest, DataConnectionTest
Bug: 181889657
Change-Id: I530180f279f2eea1b82f4e59c60c4f679625a2ce