Handle resource requests through singleton.

Many applications have multiple objects dealing with audio, and each
needs to request audio resources from resource policy manager. Resource
policy manager handles audio resources per-manager. Each ResourceSet
instance creates new manager, which causes applications having multiple
audio objects to fight for the audio resources internally, even though
the streams are played from the same process id.

To overcome this in QtMultimedia applications, handle all ResourceSet
operations through singleton. This way one client has only one manager
id registered, and resource acquiring and releasing can be limited to
only when no resources are acquired or all resources are freed. To
reduce unnecessary noise from resource policy plugin to the clients,
keep track of client states and only notify clients which themselves
request for resources etc.

Change-Id: Ifa4488a9f6298a3f601399e9d339f7bd819be757
Done-with: Juho HÃ¤mÃ¤lÃ¤inen <juho.hamalainen@tieto.com>
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>