system_server BINDER_TYPE_FD sockets using ashmem accessors

check if device is a character device, before calling
ashmem_get_size_region. We do not check if the st_rdev
matches /dev/ashmem. So this at least eliminates making
this call when associated with a socket.

Bug: 26374183
Change-Id: I68ed9d1c2cd4c47228ed065e3e18eb4151f038f4
(cherry picked from AOSP commit eab2afc7)