Otapreopt: Try to mount vendor partition for A/B OTA

Vendor apps are usually not preopted, so A/B dexopt should pick
them up. update_engine is not mounting the vendor partition, so
let otapreopt_chroot do the work.

Bug: 25612095
Bug: 29498238
Change-Id: I5a77bdb78a8e478ce10f6c1d0f911a8d6686becb
(cherry picked from commit 0ba073ce)