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
f12af5e907