ART: Don't attempt write flock if not zygote

When loading an image, don't attempt a RDWR flock if the process
is not the zygote. The lock will be rejected as the process does
not have permission to write the file.

Bug: 28011897

(cherry picked from commit fff377b1)

Change-Id: I567934c4e9a9a13e9131711240e9ecfdd58b69f7