libcutils: fallback to /dev/ashmem

For a Q APEX built with use_vendor, rather than using AIDL to talk to
ashmemd, it would directly open /dev/ashmem. R libcutils does ashmem
differently, but in order to allow R-built APEXes to replace this
category of APEXes, falling back to /dev/ashmem.

Fixes: 147363115
Test: tested on Q in swcodec APEX
Change-Id: I625e46b15fee6649251ab9be7a1bbe11c5427525