Add seperate cache path

This is set to the XDG cache path where possible.
It's set to the app's cache path on Android.

src/porting_android.h

@@ -43,10 +43,10 @@ void initAndroid();
 void cleanupAndroid();
 
 /**
- * set storage dir on external sdcard#
- * @param lJNIEnv environment from android
+ * Initializes path_* variables for Android
+ * @param env Android JNI environment
  */
-void setExternalStorageDir(JNIEnv* lJNIEnv);
+void initializePathsAndroid();
 
 /**
  * use java function to copy media from assets to external storage