Fix some track selection bugs (#649)

## Description
Fixes track selection issues with MPV. This was especially focused on
libraries that disable embedded subtitles, but also fixes issues where
the audio streams are listed before video streams.

Also includes test media source JSON and unit tests to verify many
scenarios.

### Related issues
Fixes #494

app/src/test/java/android/text/TextUtils.java

@@ -0,0 +1,9 @@
+package android.text;
+
+// Mocks static class for non-instrumented unit tests
+
+public class TextUtils {
+  public static boolean isEmpty(CharSequence str) {
+    return str == null || str.length() == 0;
+  }
+}