Properly handle last access times of 0.

Given that we filter out accesses with zero historical accesses, the
last access time should never be 0.  However, it can happen due to old
bugs.

Instead of showing a null access time, we now log a warning and hide
the usage to avoid showing null.

Test: Use dashboard normally and with last access time hardcoded to 0.
Change-Id: I306cf3b6c9334d049a6aeabd45b410eedb3580ef