Extract USB handling from AOA

The AOA code handled both USB initialization and AOA commands/events.
Extract USB-related code to a separate file and structure.

PR #2974 <https://github.com/Genymobile/scrcpy/pull/2974>

app/meson.build

@@ -78,6 +78,7 @@ if usb_support
         'src/usb/aoa_hid.c',
         'src/usb/hid_keyboard.c',
         'src/usb/hid_mouse.c',
+        'src/usb/usb.c',
     ]
 endif