Add support to get registered policy mixes

This new (Test)Api exposes all AudioMixes registered with AudioPolicyManager.
The TestApi is very useful in verifying AudioMix/AudioPolicy behavior in
the Java and Native layer to debug and assert the correct behavior.

Bug: 309080867
Test: atest AudioHostTest

Change-Id: Icdc2a482228ab06ac7386915ddbeb0cfcc24889d
diff --git a/services/audiopolicy/managerdefault/AudioPolicyManager.h b/services/audiopolicy/managerdefault/AudioPolicyManager.h
index ea60c2b..a3232a2 100644
--- a/services/audiopolicy/managerdefault/AudioPolicyManager.h
+++ b/services/audiopolicy/managerdefault/AudioPolicyManager.h
@@ -292,6 +292,7 @@
 
         virtual status_t registerPolicyMixes(const Vector<AudioMix>& mixes);
         virtual status_t unregisterPolicyMixes(Vector<AudioMix> mixes);
+        virtual status_t getRegisteredPolicyMixes(std::vector<AudioMix>& mixes) override;
         virtual status_t updatePolicyMix(
                 const AudioMix& mix,
                 const std::vector<AudioMixMatchCriterion>& updatedCriteria) override;