db/Configured: store database file in cache directory

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

android/src/Bridge.java

@@ -19,9 +19,11 @@
 
 package org.musicpd;
 
+import android.content.Context;
+
 /**
  * Bridge to native code.
  */
 public class Bridge {
-	public static native void run();
+	public static native void run(Context context);
 }