libcutils: route to /dev/ashmem<boot_id> instead of ashmemd

Having libcutils ask ashmemd for ashmem fds results in unsatisfactory
performance/memory overhead. Introduce a duplicate of /dev/ashmem and
route to it instead.

Our goal remains as before, namely, use SELinux to phase out usage of
ashmem that doesn't go through libcutils.

Bug: 139855428
Test: boot aosp_crosshatch, browse internet, use camera
Change-Id: I02260a9042acb412571b11f1f4c1d8608483064a