Deprecate a lastModified timestamp of 0 in Theme::findInCache

lastModified is used to track whether a file is outdated. Having a 0
here means we (in the old code) use the cached version. This causes
problems when the on-disk file has changed, as we have no way of
informing callers that they should regenerate the cache entry. To avoid
this, using a lastModified of 0 now warns and will always return false
(to indicate it is not cached). Unfortunately we can't simply drop the
default parameter of 0 since there is no source-compatible way of doing
that.

CCBUG: 426674