Add OTG mode

Add an option --otg to run scrcpy with only physical keyboard and mouse
simulation (HID over AOA), without mirroring and without requiring adb.

To avoid adding complexity into the scrcpy initialization and screen
implementation, OTG mode is implemented totally separately, with a
separate window.

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