[AOA FFS Implementation] Add enable_aoa_userspace_implementation flag.

This flag would enable use of AOA userspace implementation, replacing
the kernel driver, and it would be used to refactor any calls to
accessory in UsbDeviceManager.

Bug: 407985367
Bug: 416210310
Flag: android.hardware.usb.flags.enable_aoa_userspace_implementation
Test: Tested flag is available
Change-Id: Ifb7563eb12a5caf5b9cc9125a7bc5f0a1414d336