audio: Update README.md to add info about the AIDL version

This complements CL https://r.android.com/2169871.

Bug: 205884982
Test: N/A
Merged-In: Ia627e3845a078e6a4c545b13a723a6c5a038c53b
(cherry picked from commit bbf29780)
Change-Id: I78696647e64ae1a5f216c3ab4f4a7194a5145072