Add TestApis to get currently registered AudioMixes

AudioManager#getRegisteredPolicyMixes will return all AudioMixes that
are registered in the native AudioPolicyManager.

AudioPolicy#getMixes() returns all AudioMixes that are cached in the
AudioPolicy definition.

These APIs are used to help test certain conditions in GTS.

Bug: 309080867
Test: atest AudioHostTest
Change-Id: I92fdc8121f5c60bf391616ba073b0bda3b09f1cf