binder: search for receiving context pid in global namespace

BugLink: http://bugs.launchpad.net/bugs/1210149

If binder cannot find the receiving task in the sender's namespace it
will default to a null PID which thus breaks communication in our
container-based scenario. This patch extends binder to first look in the
senders namespace and if unsuccesfull search in the global namespace.

Signed-off-by: Ricardo Mendoza <ricardo.mendoza@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>