Add UsbManager @TestApi to check whether UVC gadget is enabled

This test api can be used by cts device info collection to get metrics
on how many devices support the device as webcam feature.

Bug: 372931277

Test: Check CameraDeviceInfo.json file for webcam support info

Flag: EXEMPT; bugfix

Change-Id: I12fb1c003415f82fa2cd861e1d32c9d0249299e0
Signed-off-by: Jayant Chowdhary <jchowdhary@google.com>