Maintain pre-zygote fork linear-alloc pages as shared-clean

Userfaultfd tends to dirty all the pages of the space it is used on.
However, we want to maintain the shared-clean trait of the pages
allocated in zygote process prior to first fork.

This CL separates the pre-zygote fork arenas from the userfaultfd
visited ones, thereby reataining the former's shared-clean trait.

Bug: 160737021
Test: module install and enable uffd GC
Change-Id: Iddffb2c8d2d234ce7b20c069d86341dda5443a9b
10 files changed