keep device screen off

- [x] I have checked that a similar [feature request](https://github.com/Genymobile/scrcpy/issues?q=is%3Aopen+is%3Aissue+label%3A%22feature+request%22) does not already exist.

**Is your feature request related to a problem? Please describe.**
I use the "--turn-screen-off" option, but sometimes an event (like an incoming call for example) turns it back on.

**Describe the solution you'd like**
add a `--keep-screen-off` option

or

make the `--turn-screen-off` option keep the screen always off.

**Describe alternatives you've considered**
I use the *Turn device screen off* `MOD+o` shortcut on incoming call.

Start by tracing the existing --turn-screen-off option and the MOD+o “Turn device screen off” shortcut. Determine how an incoming call or other event can turn the screen back on, then verify that the requested behavior keeps it off continuously without breaking the existing option.

Written by the indexing model from the issue text.