Do not add "adb" to the USB functions if USB is not available

emulators (cuttlefish and goldfish) don't have
USB, so they talk to adbd via vsock.

Bug: 402444824
Test: presubmit
Change-Id: I2bf4b9d85950147f177a19870334ea305088298a
Signed-off-by: Roman Kiryanov <rkir@google.com>