vold: Raise MAX_PARTITIONS to 32

 * We raised this in 10.2 globally, lets put it back to 32 so that
   new bringups don't get stuck on this. It's likely that every single
   device which uses physical, nonremovable volumes with vold will hit
   this limit.

Change-Id: Id7522681a2317c4aff49ca27e1c57e69fe57213c