Make intr optional for adb commands

All adb commands are executed with an "interruptor", so that they can be
interrupted on Ctrl+C.

Make this interruptor optional, so that we could call "adb kill-server"
in OTG mode. This command always returns almost immediately anyway.

Ideally, we should make all blocking calls interruptible (including
libusb calls, by using the asynchronous API), but it's a lot of work,
and in practice it works well enough.

PR #3011 <https://github.com/Genymobile/scrcpy/pull/3011>