SystemReader: Give a default implementation to #getTimezoneAt()

This abstract method forces subclasses (e.g. DelegateSystemReader in
gerrit) to update their code, but there is no strong reason to make it
abstract (subclasses can override it if needed).

Make the method concrete using the current default implementation
(which is the same in the mock).

Change-Id: Id1df0d71cab1d86879adf48e782f0050d3abcfa9