Only enable Pre-reboot Dexopt if version >= 2.

Version 2 contains a fix to make umount succeed.

Bug: 346869873
Test: -
  1. Turn on system tracing (Settings -> System -> Developer options -> System Tracing -> Record CPU profile).
  2. system/update_engine/scripts/update_device.py <ota-package-zip>
(cherry picked from https://android-review.googlesource.com/q/commit:ad291ba21c37f91dd80abdb294436a9e6db8f018)
Merged-In: Ie255217bb3a14648d94197020bb733d01f0733b4
Change-Id: Ie255217bb3a14648d94197020bb733d01f0733b4