Ensure that directories are initialized in GC thread.

The default directories were only being set in the backend thread, not
in the gc thread.  This only affects you if you were using the default
output directories.

Change the default temporary directory from /tmp to /tmp/ocg so that gc
doesn't try to clean up all of /tmp.

Change-Id: Ia391927ae7481371d280d945b59bd3f4a5610b1c