SystemReader#now: make it a concrete method

Abstract methods break subclasses (e.g. DelegateSystemReader in
gerrit). Updating jgit and gerrit is simpler if we do not add them. I
am not sure why some methods are abstract and others dont, but now()
can be a concrete method.

Make now() concrete. Implement it by default based on
getCurrentTime(), so subclasses overriding that method get the same
value.

Change-Id: I697749f8cba698c5388ed13ebdc2b238d6259358