Use displayName instead of operator name in MMS data notification

When the phone is in DSDS mode and the user tries to send an MMS message
from the SIM that isn't the default for data, we need to pop up a
notification letting them know that the action can't succeed unless they
turn on the advanced option to allow data use for MMS messages. This
notification was using the operator name instead of the subscription
display name, so it didn't reflect any renaming the user might have done
to keep track of their SIMs, which is obviously confusing especially if
they have two different SIMs for the same carrier. This CL switches to
using the subscription display name in this notification.

Fixes: 134771858
Test: make RunSettingsRoboTests
Change-Id: I6995bf9dd6d5e9544e26f0d8e30e97c4e73ab783