Add experimental MPV player backend (#161)

Experimental MPV player backend

Related to #14, #22, & #85

You can switch to MPV in advanced settings and toggle using hardware
decoding or not.

This uses code and buildscripts from
https://github.com/mpv-android/mpv-android, plus other third party
libraries to build MPV

app/src/main/jni/log.cpp

@@ -0,0 +1,9 @@
+#include "log.h"
+
+#include <stdlib.h>
+
+void die(const char *msg)
+{
+    ALOGE("%s", msg);
+    exit(1);
+}