Improves the changes made in 57687779 by
using AudioManager.ACTION_AUDIO_BECOMING_NOISY rather than listening for
wired headset unplug events or Bluetooth headset disconnect events. This
method is more flexible, allowing the feature to work on other types of
audio output device, as well as Bluetooth devices that don't set their
device class correctly. This change also has the benefit of being more
responsive, pausing the audio before it is rerouted to the built-in
speaker.

https://developer.android.com/guide/topics/media-apps/volume-and-earphones

Fixes the error:

 IllegalStateException: Not allowed to start service Intent { cmp=org.musicpd/.Main (has extras) }: app is in background

This seems to be required on recent Android versions (tested with Android 10).
This is also required for android TV services (cf. next commit).

This is done using Java reflection so that the project doesn't depend on
android compat libs.

MPD has minSdkVersion=21 which is above all the checks here.

- add Settings: Activity to start / stop MPD Service (Main).

- Main is a service that run in foreground with a notification. See
  Service.startForeground documentation for more details.

- Main.Client is used to control the service: start or stop it and also receive
  callbacks when service encounters an error, is killed, is started or is
  stopped.

- Main.start to start the service without any fallback.

A Java object to send logs on the android side.

The number of MPD installs on Android < 5.0 is negligible, and that
API version introduces lots of useful features for MPD.

Let the user know that MPD has failed.  Not the best thing to do, but
better than pretending it still runs.

Add class Context which wraps the Android/Java Context class and add a
JNI wrapper for method Context.getCacheDir().

Finally, MPD runs on Android.  For some small value of "runs".  Very
much work left, too much to describe.