AudioPolicyService InputDesc minor cleanup

Add a non-default constructor to set the mSessionId, and make mSessionId const.

Remove explicit clear on mEffects - it is automatically cleared by the destructor.

AudioPolicyService::setPreProcessorEnabled:
 - parameter is const *
 - use an alias instead of making a Vector copy

Destructor doesn't need to be virtual since there are no subclasses.

Change-Id: Ibc3c3bea8259839430b1cf5356186c7d96f1082f