ueventd: duplicate /dev/ashmem

We want ashmem to only be used via libcutils API, with long-term goal
being deprecation of ashmem with memfd. To do that we route libcutils to
a new source of ashmem fds. We then phase out uses of /dev/ashmem that
doesn't go through libcutils using SELinux.

In Q, we introduced ashmemd as the source of ashmem fds to libcutils.
However, having a separate process and, consequently, binder hops to
handle /dev/ashmem results in performance/memory overhead.

To address the overhead, replace ashmemd with a duplicate of
/dev/ashmem. Name it /dev/ashmem<boot_id>, where boot_id is a random
number generated on each boot. This way we make sure that developers
don't accidentally depend on /dev/ashmem<boot_id>, as that name can't be
hardcoded.

Bug: 139855428
Test: writing "add"/"remove" to /sys/class/misc/ashmem/uevent correctly
adds/removes /dev/ashmem and /dev/ashmem/boot_id
Change-Id: I36d23116048bfcd99903ba46cc133161835a2cfa