Unstable bluetooth connect workflow

[SebaDro]

Description
The workflow for connecting to bluetooth devices is unstable, as sometimes the app crashes after re-connecting to a removed device or a device occurs multiple times in the list of paired devices.

How to reproduce
For crashing the app do the following:

1. Delete a device from the list of paired devices
2. Click on the back button
3. Go back to the OBD selection fragment
4. Try to pair with device you have deleted before
   => app crashed

For multiple occurence of the same device do the steps below:

1. Pair with a device
2. Delete the device from the list of paired devices
3. Rediscover devices
4. Try to pair with the device you have deleted before
   => device occurs multiple times in the list of paired devices

[SebaDro]

Fixed in parts by #847

[SebaDro]

The app still crashes when performing the above described actions. Occuring issue: when resuming to the OBDSelectionFragment the context changes. But inside the BluetoothDevicePairingCallback the context surprisingly is null which leads to a failure when calling getString():

enviroCar-app/org.envirocar.app/src/org/envirocar/app/views/obdselection/OBDSelectionFragment.java

Lines 531 to 533 in 308f3db

	showSnackbar(String.format(
	getString(R.string.obd_selection_pairing_success_template),
	device.getName()));

Unfortunately, no solution found so far.