OatFileAssistantTest: do not relocate image randomly.

Manually relocate the boot image to a known location before starting
the runtime so that the image won't be randomly relocated. This fixes
flaky test failures due to the image randomly being relocated by 0.

Bug: 28294887
Bug: 29557002

(cherry picked from commit a48403eb)

Change-Id: I05a7961f1237a05e89d3f631c959e260ccfe0b70