Fix adb server hang

Since commit 04267085441d6fcd05eff7df0118708f7622e237, the server is run
in a dedicated thread. For SDL, many signals, including SIGINT and
SIGTERM, are masked for new threads. As a result, if the adb server is
not already running, adb commands invoked by scrcpy will start an adb
server that ignores those signals and cannot be terminated at system
shutdown.

Fixes #2873 <https://github.com/Genymobile/scrcpy/issues/2873>
PR #2870 <https://github.com/Genymobile/scrcpy/pull/2870>

Signed-off-by: Romain Vimont <rom@rom1v.com>