staging: binder: Change binder mutex to rtmutex.

Surfaceflinger uses binder heavily to receive/send frames from applications
while compositing the screen. Change the binder mutex to an rt mutex to minimize
instances where high priority surfaceflinger binder work is blocked by lower
priority binder ipc.

Signed-off-by: Riley Andrews <riandrews@google.com>
Change-Id: I086a715267648448f0c5f62b037a3093d1079a79