[Sb refactor] Rework TelephonyCallback flows

MobileConnectionRepository was doing a 1:1 mapping from its single
TelephonyCallback listener to the `connectionInfo` model that it emitted
from its flow. This wasn't great because there wasn't an easy way to add
new callbacks which emitted from flows _other than_ the `connectionInfo`
flow. In our case, we wanted `dataEnabled` to subscribe to the callback
without starting collection on the `connectionInfo`. Secondly, we can
now have a better polling event which is triggered on any telephony
callback instead of the `onEach` approach which was used before (see
`telephonyPollingEvent` for the change)

Test: tests in com.android.systemui.statusbar.pipeline.mobile
Bug: 238425913
Change-Id: I6843e949179f429eb8271e4021f884fbc079bf09