Strings for "Data during calls" preference

In some dual-SIM cases, when you have an active voice call using a SIM
that isn't the default one for mobile data, you may not get data
connectivity at all (from any other SIM) due to technical limitations in
the telephony stack. This preference will allow the user to leave data
off for a SIM, but opt in to have data become active only during calls.

Bug: 132114205
Test: n/a
Change-Id: I3e7cdaedd8b685e0fe65221d94f5b426bc95942e