Cope with angler's whitespace-padded partition size reporting.

Before:

  $ fastboot -w
  wiping userdata...
  Couldn't parse partition size '0x        0x66257ee00'.
  wiping cache...
  Couldn't parse partition size '0x        0x6400000'.
  erasing 'userdata'...

Bug: http://b/25653580
Change-Id: I301b8410689c1e52681796c240a149d270360edf