Expose playback controls to Operating System

Environment: pureos 10, phosh 0.22.0, shortwave 3.1.0 flatpak install, L5 mobile, arm64, pulseaudio 14.2.

What is happening: no persistent playback control on device lockscreen, or in phosh notification area. Once user selects pause on exposed playback control notification disappears and user has to navigate to shortwave app and hit the play button. No persistent playback control when using bluetooth on device such as the pinewatch. When pause is selected from shell stream cannot be played through bluetooth play button control.

What it should show: play, pause, next station? (since songs cant be skipped).

Related: #471 not sure its exact same.

Tested on bluetooth and controls are exposed and work when pausing playing from remote control.

Here is a RUST backtrace when using the notification widget in phosh to pause Shortwave, followed by the disappearing notification: