Fix SharedPrefsBackupHelper so it doesn't hard code the paths to the files.

This took quite a bit of refactoring.

test-runner/android/test/mock/MockContext.java

@@ -104,6 +104,11 @@ public class MockContext extends Context {
         throw new UnsupportedOperationException();
     }
 
+    @Override
+    public File getSharedPrefsFile(String name) {
+        throw new UnsupportedOperationException();
+    }
+
     @Override
     public String getPackageCodePath() {
         throw new UnsupportedOperationException();