Drop root privileges on startup

On rooted devices, switch to user 2000 during startup.

Copy-paste does not work as root (user 0), so switching to 2000 fixes
the issue.

Fixes #6224 <https://github.com/Genymobile/scrcpy/issues/6224>

Suggested-by: Simon Chan <1330321+yume-chan@users.noreply.github.com>