GattManager issues

Android GATT callbacks appear to be executing on arbitrary Binder threads.  I think `GattManager#onCharacteristicChanged()` needs to be synchronized to avoid concurrent access to `mCharacteristicChangeListeners`.

Also, this:

```
public BluetoothGatt getGatt(BluetoothDevice device) {
    return mGatts.get(device);
}
```

Should be `mGatts.get(device.getAddress())`.
