[AOA FFS Implementation] Verify FFS directory for AOA

This commit checks if the accessory directories exist and are mounted as FFS. If these conditions are met it will enable the AOA userspace implementation.

Bug: 407985367
Bug: 424156976
Flag: android.hardware.usb.flags.enable_aoa_userspace_implementation
Test: manual check the presence of folders
Test: Run CTS USB test cases with enable_aoa_userspace_implementation enabled
Change-Id: I34270582dd2d0c971a12c746ba563e8fb1c61861