Avoid IllegalArgumentException in Preference.java compareTo()

In some situations the collection.sort() algoritm fails in compareTo()
with a: java.lang.IllegalArgumentException: Comparison method violates
its general contract!, due to a more strict validation of the compare
contract.
This strict validation was introduced in java 1.7.
See http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6804124
This fix should prevent the sorting from crashing.

Change-Id: I64230e7f62f83c99d7a6274964bb2988ae843826