audio policy: session routes continued.

- Clear session routes when client process dies.
- Enforce the route only when the requesting session is active.
- Fix requested route not working if an output mix change is required
(e.g forcing to SPEAKER when the default route is USB or A2DP).
- Make sure all sessions sharing the strategy with a rerouted session
have the same route (needed for volume control consistency)

Change-Id: I0ab347a8fb97e73e2c5965374544c5f4fe509ef1