Improve AudioMix registration/unregistration

1) Pass through the AudioMix binder token introduced in frameworks/base
   all the way to AudioPolicyManager incl. aidl conversions.
2) When AudioPolicyManager#registerPolicyMixes fails improve the
   resetting logic. Only unregister policy mixes that were not already
registered before the call to registerPolicyMixes.
3) AudioPolicyManager#unregisterPolicyMixes returns an error if any
   mix unregistration failed. Previously the method only returned an
error if the LAST unregistration failed.
4) AudioPolicyMix#unregisterMix matches mixes on the AudioMix binder
   token instead of device type + address. This ensures AudioMixes are
uniquely identified and different policies cannot interfere with each
other.

Bug: 309080867
Test: atest AudioHostTest
Change-Id: If56134c56d77c615ed40e039aa566032b445842d