Correct behavior of "Mobile data" button on opportunistic subscriptions.

A check was added earlier to make sure in DSDS mode only one sub
shows data on, which is not true for opportunistic subscriptions. For
them, it should always be on (enforced by Telephony) and be disabled
as it shouldn't be turned off.

Bug: 124331860
Bug: 124683219
Test: manual, robolectric
Change-Id: I53c8d881ff71ad25c418085d3eee63e55c786b8f