FolderMemoryCard: Implement GetCRC() via a timestamp of the last memory card write.

Reasoning:
Regarding auto-ejecting on save load, I see that the current
implementation checks that by comparing memory card CRC and reinserting
if it mismatches. Since it's actually just about seeing if the memory
card state of the savestate and the current state match, my GetCRC() now
returns a millisecond timestamp of the last time the card was written
to. This should work out to the intended result, though I had to use
wxGetLocalTimeMillis() instead of wxGetUTCTimeMillis() since the latter
isn't available for some reason.