Android: Set "exported" manifest setting to false

[haukepribnow]

Sets the Android manifest setting android:exported to false. This seems to ensure that only the "parent" app will be able to interact with the background service - and not any other app.

By interpreting the partially Japanese translation of the original commit message, this seems to be a finding of MobSF.

FYI: I have already applied the same PR to my own repository: https://github.com/haukepribnow/background_location/pull/2