Add com.android.future.usb.accessory api files

API txt files for com.android.future.usb.accessory are added.
The contents are empty as they didn't exist before API level 28.

Bug: 77577799
Test: make -j
      mkae checkapi
      make update-api

Change-Id: I21fdcde26dbff033e0d46b31a0cd3db3bd738642
6 files changed