Decrease OTA package verification times further.

Timing from Nexus 5X:

      89 MiB OTA update package: 1.4 s -> 0.6 s (decreased by 57%)
    1196 MiB OTA update package: 8.0 s -> 7.5 s (decreased by 6%)

Bug: http://b/28135231
Change-Id: Id91f2ad15df2bffb9f8a4b4ec5a57657a02847ec