[AOA FFS Implementation] Add accessory legacy bridge

Contains the implementation of async IO for piping USB reads from FFS to
socket and pipe writes from socket to USB FFS.

This will be used in a followup change.
Bug: 407985367
Bug: 416211970
Flag: android.hardware.usb.flags.enable_aoa_userspace_implementation
Test: No build breakage
Change-Id: If37a3f97d8ea52cf2f02207a29d662256d94bc89