Fix two problems related to data connectivity in the multi-SIM header

When you have multiple active SIMs, the Network & internet page has a
header showing entries for each one, with summary text indicating which
one is used for data (and whether it is just set as the default, or
actively using data). We were not properly setting this text when either
data wasn't being used, eg when connected to wifi, or mobile data was
disabled for this SIM. This CL fixes both these problems by adding new
helper classes to listen for relevant events.

Test: make RunSettingsRoboTests
Fixes: 124394250
Fixes: 128857712
Change-Id: I34f2679752fa41a50247dd0b12581cbfd77a34f6