Adding notification channel to support devices with Android 8.0 and above

[anuvakkachen]

This PR adds a notification channel to be used in notifications posted by BluetoothMedic.
Posting notifications without a channel does not work on Android 8.0 and above

Expected Behavior:
When BluetoothMedic is used for power cycling bluetooth, on scan failures, and Medic notifications are enabled, when a scan failure occurs , bluetooth is power cycled and a notification is posted that says so.

Actual Behavior:
Bluetooth is power cycled, but no notification is posted.
If Settings> developer options > Show Notification channel warnings is turned on, a warning toast message can be seen, .

Steps to reproduce:

1. Use the android beacon library reference app and enable BluetoothMedic power cycling and notifications in RangingActivity

   BluetoothMedic.getInstance().enablePowerCycleOnFailures(this);
   BluetoothMedic.getInstance().setNotificationsEnabled( true, R.drawable.ic_launcher);

2. Broadcast an 'onScanFailed' to simulate an LE scan failure

   Intent intent = new Intent("onScanFailed");
   intent.putExtra("errorCode",2);
   LocalBroadcastManager.getInstance(RangingActivity.this).sendBroadcast(intent);

3. Run RangingActivity

We should be able to see a bluetooth power cycling but no notification from BluetoothMedic

Device used : Galaxy Note 8
Android version : 9.0

Attaching screenshots for

before fix

after fix

[davidgyoung]

Thank you for this pull request, @anuvakkachen. This looks like a very useful and necessary change.

Apologies for taking so long to review this. I will test within the next few days and let you know if any changes are needed before merging.

[davidgyoung]

This is now in a 2.16.4 release

[anuvakkachen]

Thank you for merging this PR @davidgyoung