Fix image_space_test on Q+ target.

This is a follow-up after
    https://android-review.googlesource.com/958079
to make image_space_test pass when DEX2OATBOOTCLASSPATH is
set, i.e. on a Q+ device.

Test: image_space_test
Bug: 122937705

(cherry picked from commit 3f795d2ab38ede7821db73904ddd1ddadb6bac4e)

Change-Id: I237ffd348a4466981b969ff1c7fc93a9b4a1dc44
Merged-In: If84cde5e68ae42aed7d2a402817436c19ee6fc21