[Sb] Add prioritize_latency tracking in mobile connections

This CL adds a network callback -- registered via ConnectivityManager --
for every mobile subscription that we know about. This amounts to one
listeners per subId (more on that below). The network callback is
registered only to receive information about _that_ subId's network
pertaining to the NET_CAPABILITY_PRIORITIZE_LATENCY. We'll use this flag
downstream to determine whether or not to show a network slice
attribution.

On ConnectivityManager's callback list:
  The docs on ConnectivityManagerService claims that we should be careful
about too many callbacks registered with the system. If we need to get
rid of the 1:1 mapping from subscription to registrant, then we would
have to do some work to register a _single_ callback that maintained a
map of <netId, networkCapabilities>. It would also presumably have to be
eagerly listening, and make sure to update the map whenever a network
disappears
  Note that this doesn't actually give us a huge performance benefit
from what I can tell. This is because our single registrant would still
get a callback-per-network in which the capabilities changed. Therefore,
we still get the penalty of N incoming IPCs when there are N networks
that meet the criteria. So we only should worry about that in the case
where we run close to the limit of 100 subscribers per process.

Test: tests in statusbar/pipeline
Bug: 270385675
Change-Id: If42b52ad6e14bdc258c90761e3e6dd629bbe9d3d