Add a convenience method to facilitate tests

In app tests (as opposed to RoboTests), we don't have a shadow
BluetoothSettings class and can't mock it because it's final, but we
need a way to mock out the address of a CachedBluetoothDevice. So this
adds a convenience method on CachedBluetoothDevice that can be mocked.

Bug: 62447414
Test: (should be covered by other tests)
Change-Id: If818d3dcc783b4aa03f7c6d7c441ba44ea980ab7