ashmem: fix deadlock when system is low on memory

When system is low on memory, allocating memory while holding the
ashmem_mutex may try to directly reclaim memory. Then ashmem_shrink()
is called in same thread. It will deadlock at acquiring ashmem_mutex.
This change lets ashmem_shrink() return failure if ashmem_mutex is not
available. Memory will be reclaimed from others.

Change-Id: I3b6024d5477d37245f4ebde6d7069fb77af789ee
Signed-off-by: Yuanyuan Zhong <yuanyuan.zhong@gmail.com>