The AAC option is there so you can directly pass AAC to the headphones without re-encoding it first. Android can live encode AAC, but is only used in cases where you need to mix audio (such as notification or ring sounds while playing AAC source material).
All of their testing involves forcing AAC on all the time, even if Android would have never selected it. Android selects whatever the best codec for the specific case is: if your audio is already AAC, it prefers AAC; else for all other codecs, it prefers AptX HD, LDAC, AptX, then SBC (depending on what your phone and your headphones support).
iOS also prefers it the same way: if already AAC, use AAC, else use the best codec you can negotiate that isn't AAC.
Nothing can beat the quality of not transcoding something in the first place. Most audio on the Internet is already available in AAC.