fix($parse): stabilize one-time literal expressions correctly

Change `oneTimeLiteralWatchDelegate` (introduced in c024f28) so that it implements the stabilization algorithm described in the expression docs, like `oneTimeWatchDelegate` does.

At the end of the digest, the delegate must look at the *last* value seen by the expression. Otherwise the expression becomes stable even when the array/object members go back to `undefined` during the digest.