Increase OCG warning/critical space thresholds.

We're working on tracking down a space leak here.  But in the interim,
there's no reason to have these thresholds so low: the machines in
question have 500G disk, of which 453G is currently free.  (And 32G ram
disk.)

Bug: 71341
Bug: 71260
Change-Id: I094fcd5d46a1a97acafd75484d0b5959c08448e7