Normalize WatchedItem expiry field
Normalize watchlist expiry field so that it is set to
ConvertibleTimestamp and the expiry value validation is
handled by the WatchedItem class.
Bug: T260868
Bug: T260009
Change-Id: I3ef31900cfbe7bce23c5ebe1db777a5137ea6167
(cherry picked from commit 45bb0271221991f7edfbff5181f7b8de11c18e19)