audio: Add @export annotations to microphone-related enums

This addresses long-standing TODOs in system/audio.h.
Annotated the following enums:

 - AudioMicrophoneChannelMapping;
 - AudioMicrophoneLocation;
 - AudioMicrophoneDirectionality.

This allows removing hand-written conversion functions
since the values are guaranteed to match between HIDL and
system/audio.h.

Bug: 122858783
Test: m
Change-Id: I2bf250ef9154a7ae86679afa566c80dfd01f1d5b
Merged-In: I2bf250ef9154a7ae86679afa566c80dfd01f1d5b