Fix SharedBuffer. Remove aref.

Add comment that SharedBuffer is deprecated.

Both aref and SharedBuffer had memory ordering bugs.  Aref has no
clients.

SharedBuffer had several bugs, which are fixed here:

mRefs was declared neither volatile, not atomic, allowing the
compiler to, for example, reuse a stale previously loaded value.

It used the default android_atomic release memory ordering, which
is insufficient for reference count decrements.

It used an ordinary memory read in onlyOwner() to check whether
an object is safe to deallocate, without any attempt to ensure
memory ordering.

Comments claimed that SharedBuffer was exactly 16 bytes, but
this was neither checked, nor correct on 64-bit platforms.

This turns mRef into a std::atomic and removes the android_atomic
dependency.

Bug: 28826227
Change-Id: I39fa0b4f70ac0471b14ad274806fc4e0c0802e78
(cherry picked from commit 3e4c076e)