media-keys: Use heuristic to switch MPRIS clients

Instead of holding on to an MPRIS client for its entire lifetime, switch
to another client when it starts playing or when it first appears on the
bus, but only if the current client is not itself playing.
6 jobs for rhendric/current-mpris-client-heuristic in 2 minutes and 49 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #806322
build

00:00:40

 
  Test
passed #806323
test

00:02:09

 
  Manual
manual #806324
privileged allowed to fail manual
asan
manual #806327
allowed to fail manual
msan
manual #806325
allowed to fail manual
tsan
manual #806326
allowed to fail manual
ubsan