Reset WatchedItemStore default instance after tests

Reset WatchedItemStore default instance after tests

Prior to this change in tests the overridden store
would remain in the instance static and thus could
be used in other places.

This patch introduces the used of ScopedCallbacks
in the override methods in WatchedItemStore.
This means that any instance of WatchedItemStore
should return to a regular state after each test.

This is better than requiring the tests to reset
the value back to the origional as this would likely
be forgotten and result in long hunts for failing
tests.

This was found while writing more tests...

Change-Id: I9aa71425642174ae9ea2c6d4f85dcd07d724af11