Fix FileSnapShot

Fix FileSnapShot

We cannot use SystemReader to get the time, unless we do that consistently,
which is harder to do and be sure we are really testing what we want.

Then we need to update our lastRead variable whenever we conclude that
our file is not racily clean according to lastRead. It may well be clean,
but we do not know that until we check the system clock again.

Finally add a test for this class.

Change-Id: I1894b032b9bd359d1b5325e5472d48e372599e4c
Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>