Allow injection of clock into ExpiringMemoizationSupplier to allow for testing

[mokah]

To allow testing of ExpiringMemoizationSupplier, a second constructor with @VisibleForTesting should be added that allows the injection of a clock.

Right now it is being tested by calling Thread.sleep(x) but an injection of a clock would be a better way to test, I believe. Thoughts?

[cgdecker]

It could be a slight improvement, perhaps, but I'm not sure it's worth it in this case.

The main reason I say that is because ExpiringMemoizingSupplier is Serializable, and changing it to have a Ticker field to allow for this testing would also require making our system time Ticker serializable... I don't know that that's something we'd want to do, because it doesn't have any state or data to actually serialize and instead just relies on the system clock.