Add support for memfd

This patch adds support to make libcutils create and use memfd where
possible, instead of ashmem.

The libcutils library is made dynamic, since apps are still sending over
ashmem FDs to processes and we are working on moving those apps away
from ashmem. The best we can do for that is make the library work, and
force apps in the long run to move away from directly using ashmem.

Due to further complications mentioned in the 'NOTE' section of the
patch, memfd is kept disabled for now, so nothing changes yet. To enable
it, set the sys.use_memfd property (check user_debug build).

Bug: 113362644
Change-Id: I491cb7841b664db5b1c26b3e07a440a14810e182
Signed-off-by: Joel Fernandes <joelaf@google.com>