[AOA FFS Implementation] Add Vendor Control Request Monitor

Adds a thread which will monitor a given FunctionFS endpoint for control
requests, and explicitly handle vendor control requests which are
received over USB. This thread provides generic vendor control request
handling. This CL only adds the class, it will be used in a later CL
(ag/33856076) and the usage of the thread will be flagged.

Bug: 407985367
Bug: 416246384
Flag: android.hardware.usb.flags.enable_aoa_userspace_implementation
Test: No breakage
Change-Id: I76edd0d126b155c19c29828c12162df1eb8a1d45