Fix a SIGSEGV at background-to-foreground collector transition.

The mem map that's used to create the backup main rosalloc space is
set to PROT_NONE by the compaction and the rosalloc space creation
attempts to write the debug magic number in the first page.

This is a debug build only issue.

Cherry pick commit c1276c838d1d0896048b8c14458432b94274e869.

Bug: 16854263
Change-Id: Ide2545d92bcafd3a095ac7f0c89c5b43d6dd78de
1 file changed