Providing a way for user to allow MMS data even if mobile data is off.

We are providing a way for user to allow MMS usage even if mobile data
if off. Earlier, MMS is sometimes considered metered and therefore
disallowed if user turns mobile data off.

Bug: 128616035
Test: unittest
Change-Id: I64d0b7f271858d60f8c292af0aa5371ad21d9805