Make service stopping more reliable

[davidgyoung]

Several changes to make the library behave properly when starting and stopping a foreground service on Android O+, and properly switching back to scan jobs when the foreground service is not active.

• Add a stopSelf() call when the last consumer is unbound from the scanning service to attempt to make service destruction (and force stopping scanning) more reliable. Call stopForeground() before the call to stopSelf() to prevent a crash.
• Stop intent based scanning whenever stopping the ScanJob, so it is not left on (potentially launching Scan Jobs again on detection even when the BeaconService is active in the foreground).