Allow mmap for vendor_init

vendor_init needs to touch a bunch of files. Forgotten within this set
of permissions is the ability to mmap files.

Addresses the following denial:

  avc:  denied  { map } for  pid=1167 comm="init" path="/system/etc/selinux/plat_file_contexts" dev="vda1" ino=1845 scontext=u:r:vendor_init:s0 tcontext=u:object_r:file_contexts_file:s0 tclass=file permissive=0

While I'm here, add mmap() support to other areas where it's likely
needed.

Bug: 111742629
Test: make -j80, ran emulator
Change-Id: Icab00e45ae88f0d86be66d85a22e018af6ffcd75