audiopolicy: Remove raw pointer references to AudioMix

AudioInputDescriptor and AudioOutputDescriptor 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 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 crosshatch,
      build crosshatch with USE_CONFIGURABLE_AUDIO_POLICY := 1
Change-Id: Ic508caedefe721ed7e7ba6ee3e9175ba9e8dc23a