Lower flock LOG(FATAL) to LOG(WARNING)

Failing to unlock a file should not be a fatal error.

The lock will usually be cleared right after this anyway by the file
being closed. Even if it isn't though this should not cause deadlocks
since most places either don't retry if they don't get the lock the
first time or are separate binaries (such as dex2oat) that are run
with external timeouts.

Bug: 36369345
Test: ./test.py --host -j40
Change-Id: Icd783c038de3c263805e8de1bbc35b2ee8918340
(cherry picked from commit 9c48ee5b)