AAudio: Add Doxygen configuration file

Added files:
Doxyfile.orig - corresponds to default Doxyfile for Doxygen version 1.8.6
Doxyfile - Doxyfile.orig modified for Oboe

Other related changes:
Corrected Doxygen comment typo in AAudio.h
Added instructions in README on how to use Doxygen

Bug: 35255705
Test: run doxygen in same directory as Doxyfile
Change-Id: I9f431474a17b9e87b8e75347a50f561344204d83