[Sb refactor] Fix missed telephony callback events

This change implements a new flow operator, `coalesce`, whichs allows
for us to design a backpressure strategy in
`MobileConnectionRepositoryImpl` such that we never drop a
`TelephonyCallback` event if it is the only one of its type.

In practice, this means that if we have a late subscriber to a field
which is backed by the `callbackEvents` flow, we'll never miss data that
would otherwise have been dropped due to unrelated callbacks before the
subscriber shows up.

Test: tests in statusbar/pipeline/mobile
Test: MobileConnectionTelephonySmokeTests
Fixes: 272784824
Change-Id: Id57cc4047d9290581e6b8eae813136ebfb8eb3be