RRO: Synchronize access to overlays.list

idmap --scan is executed as a part of the pre-loading in ZygoteInit.
The pre loading is executed in parallel for each supported architecture
(32/64 bit). This will cause a race condition in the creation of the
overlays.list file and the idmap files for the system overlays.

Apply flock on overlays.list to prevent the file from being thrown away
and recreated when it is in use by another Zygote.

Bug: 28032298

Change-Id: I51d39f121d207b11181340b68b164b60020f0c61