aries | p1: ashmem: fix ashmem_shrink deadlock

Don't acquire ashmem_mutex in ashmem_shrink if we've somehow recursed into the
shrinker code from within ashmem. Just bail out, avoiding a deadlock. This is
fine, as ashmem cache pruning is advisory anyhow.

Signed-off-by: Robert Love <rlove@google.com>
Signed-off-by: Humberto Borba <humberos@gmail.com>
Change-Id: Icbc976f44a41796b5256eb75a61dcaae6c501781