DO NOT MERGE: audiopolicy: Remove raw pointer references to AudioMix

AudioInputDescriptor, AudioOutputDescriptor, and AudioSession used
to reference AudioMix instances using a raw pointer. This isn't
safe as AudioMix was owned by AudioPolicyMix, which is not
referenced by descriptors.

Change AudioMix* pointers in Audio{Input|Output}Descriptor and
AudioSession to wp<AudioPolicyMix> which reflects their
relationship correctly.

To ensure that code does not operate on AudioMix instances
independently from AudioPolicyMix, and to avoid introducing
a lot of getter / setter methods into AudioPolicyMix, make
the latter to inherit AudioMix. This makes sense because
AudioPolicyMix is essentially a ref-counted version of AudioMix.

Bug: 124899895
Test: build and sanity check on marlin,
      build marlin with USE_CONFIGURABLE_AUDIO_POLICY := 1
Merged-In: Ic508caedefe721ed7e7ba6ee3e9175ba9e8dc23a
Change-Id: Ic508caedefe721ed7e7ba6ee3e9175ba9e8dc23a
(cherry picked from commit a306e2a3)