[AOA FFS Implementation] Fetch accessory strings from FFS

Accessory mode needs to fetch the accessory strings associated with
connected accessory. The kernel driver used to provide these strings to
UsbDeviceManager. The same behaviour needs to be replicated by getting
the accessory strings from the newly created vendor control request
monitor. UsbDeviceManager fetches from /dev/usb_accessory or from vendor
control request monitor based on the flag.

Bug: 407985367
Bug: 422137361
Flag: android.hardware.usb.flags.enable_aoa_userspace_implementation
Test: Checked that accessory strings are fetched from userspace
Change-Id: I55fdbac0d0b2e4067050d3c85d2f7c6729586b6b