Add VPX output buffer size check

and handle dead observers more gracefully

Bug: 27597103
Change-Id: Id7acb25d5ef69b197da15ec200a9e4f9e7b03518

status_t OMX::useBuffer(
        node_id node, OMX_U32 port_index, const sp<IMemory> &params,
        buffer_id *buffer, OMX_U32 allottedSize) {
 return findInstance(node)->useBuffer(
            port_index, params, buffer, allottedSize);
}

status_t OMX::useBuffer(
        node_id node, OMX_U32 port_index, const sp<IMemory> &params,
        buffer_id *buffer, OMX_U32 allottedSize) {
 return findInstance(node)->useBuffer(
            port_index, params, buffer, allottedSize);
}
