Change signalStrength to int.

Broadcast signal strength is calculated differently than unicast
so the SignalStrength class used for unicast doesn't make sense.
Also added a -1 constant for unavailable which is mainly used
when the service is delivered via unicast.

Also rename signalStrengthUpdated to broadcastSignalStrengthUpdated.

Test: builds
Change-Id: Ia2680370d4cb8cd838341f58b41d02cdc78d4fc1