save app context to avoid leaking activity

vtm-android/src/org/oscim/android/AndroidAssets.java

@@ -1,5 +1,6 @@
 /*
  * Copyright 2013 Hannes Janetzek
+ * Copyright 2016 Stephan Leuschner
  *
  * This file is part of the OpenScienceMap project (http://www.opensciencemap.org).
  *
@@ -31,7 +32,7 @@ public class AndroidAssets extends AssetAdapter {
     }
 
     private AndroidAssets(Context ctx) {
-        mContext = ctx;
+        mContext = ctx.getApplicationContext();
     }
 
     @Override