Closes #396

The flag `-stdlib=libc++` doesn't appear to work because it attempts
to load `libc++` and not `libc++_static`, and it omits `libc++abi`.

So long, autotools!  This is my last MPD related project to migrate
away from it.  It has its strengths, but also very obvious weaknesses
and weirdnesses.  Today, many of its quirks are not needed anymore,
and are cumbersome and slow.  Now welcome our new Meson overlords!

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

add 2 preferences to:
 - enable Wakelock when MPD is running (prevent suspend)
 - run MPD on boot

and display MPD logs

 - <uses-permission> must be before <application>
 - specify allowBackup (default)

- 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.

First commit for issue #69

Let FFmpeg do the MP3 decoding.  See commit
a4de9650

Enable the NFS storage plugin on Android.

Closes #226

Closes #213.

Apparently, clang defaults to NEON when ARMv7 is used.  Not all ARMv7
CPUs we target have NEON, so we need to disable that.

Android native code should be position-independent.

The NDK build scripts use "-fpic" instead of "-fPIC" for ARM, but that
doesn't work with FFmpeg's assembly code, because it requires
R_ARM_MOVW_ABS_NC which is unavailable with "-fpic".

By telling clang which implementation to use, we avoid the dependency
on libstdc++.so.

.. and make all library symbols hidden by default.

Saves big amounts of .text section size with --gc-sections, because
only this allows discarding unused functions from those (static)
third-party libraries.