Partially fixes #278
Move sharedPreferences keys from strings.xml to keys.xml.
Remove keys from Constants.java.
Implement Keys.java to read keys from the xml file.
Use methods in Keys.java instead of hard coding the key strings
or duplicating them in Constants.java.
This is not an ideal solution, as we need a Context to look up the
key values in the resource file. A Context is not always available,
and makes the code ugly.
Will refactor into a better solution before submitting pull request.