Don't hold a lock while reading shared preferences from disk.

Shared prefrences loads thir content from disk on a separate
thread to improve performance, however it holds the lock
the whole time  while reading from disk which as a result blocks
operations that don't rely on reading data from being performed
intil load completes, e.g. reguistering a prefernces change
listener does not depend on having the data loaded.

bug:5254577

Change-Id: I5ad67b285631c34d5aadac7138ba8bfaa728cf94