Integrate with mediakeys dbus extension (MPRIS)

- [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.

Thanks for scrcpy! I'm always surprised how seamless this tool works!

**Is your feature request related to a problem? Please describe.**
With the introduction of audio forward in scrcpy 2.0, I want to start using my smartphone as my main media player. While sound and mouse/keyboard control works nicely I wished that it could integrate with my Linux Desktop even better.

**Describe the solution you'd like**

It would be awesome if scrcpy could interact with the [MPRIS](https://specifications.freedesktop.org/mpris-spec/latest/) interface to support media buttons and maybe even report the current play state/artist/title from android (I'm not sure if that's possible though)

**Describe alternatives you've considered**

Currently I'm using the spotify webinterface as kind of "remote control" to make the media buttons work on linux. While this is enough for my use case, it is clumsy and only works with spotify.

Start by reading the MPRIS specification and the audio-forwarding behavior described in the issue; no source file or test is identified. Define which media-button actions and Android playback metadata are in scope, then validate the integration against those agreed behaviors.

Written by the indexing model from the issue text.