ObservableServiceConnection introduction.

This changelist introduces ObservableServiceConnection,
a concrete implementation of ServiceConnection that can
be observed by multiple clients for service status
change. The connection also is typed transforming the
resulting proxy into an internal type.

Bug: 202758097
Test: atest ObservableServiceConnectionTest
Change-Id: I06710531c77724d81add07291d5f52b34aa36498