Rename exported API and improve the documentation

1. Export the MIX_ROLE_* instead of the internal used MIX_TYPE_*
2. Rename API from "type" to "role"
3. Improve the documentation

Bug: 183693610, 194163617
Test: atest com.google.android.gts.audioservice.AudioServiceHostTest
CTS-Coverage-Bug: 191716676
Change-Id: I3f57a832751814469fce4686290212bfda5bd199