system_server BINDER_TYPE_FD driver ashmem accessors

check if device matches the ashmem rdev, before calling
ashmem_get_size_region. This eliminates making this call
when associated with other driver file descriptors.

Bug: 26374183
Bug: 26918423
Bug: 26871259
Change-Id: I1f88c2c93ea35a73c8e14125f3d1a6c67fa4f15b