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

(cherry picked from commit a2db2618)

Change-Id: I15e6bee4d88695d4482013c4f86586276acdea84