[policy] Do not route to A2DP if call exists.

Even if call resource class is not active, if call is in any state other
than inactive, do not route audio to A2DP. This way we can prevent audio
from leaking to A2DP when ringtone playing in incoming call state ends
before call state changes to active.
Merge pull request #4 from jusa/call-dont-route-a2dp

[policy] Do not route to A2DP if call exists.