Show signal strength of current active subscription

For grouped primary and opportunistic use-case, if both are active,
mobile signal bar should by default show which one is currently
used for Internet data. Unless carrier set it otherwise -- if they
want to always show signal bar of primary subscription.

Bug: 123780907
Test: manual
Change-Id: I62e298e7b9bdc82dd44f0c8ae783c623e101b0ef