Add mpris-proxy to user session

Allows to control music playback from e.g. a cars entertainment unit and
shows artist information.

debian/librem5-base-defaults.mpris-proxy.user.service

+[Unit]
+Description=Bluetooth mpris proxy
+Documentation=man:mpris-proxy(1)
+
+Wants=dbus.socket
+After=dbus.socket
+
+[Service]
+Type=simple
+ExecStart=/usr/bin/mpris-proxy
+
+[Install]
+WantedBy=default.target

debian/rules

@@ -19,6 +19,9 @@ override_dh_installsystemd:
 	dh_installsystemd --no-start --name=bluetooth-brcmfmac
 	dh_installsystemd --no-start --name=librem5-lockdown-support
 
+override_dh_installsystemduser:
+	dh_installsystemduser --name=mpris-proxy
+
 override_dh_auto_test:
 	desktop-file-validate default/hidden-apps/*.desktop