Treat default value that wasn't found as empty value.

Fixes http://code.google.com/p/android/issues/detail?id=33220

Change-Id: If22e8b245f688d922c168aa54c8acf743c0644d7
Factor out preference-in-storage check to overridable method.

Change-Id: I2fff3192802f8a180330fda66388eda394334a5a