job: Add JobManagerInterface to emit JobStarted signal

This adds the JobManagerInterface helper object to emit a signal when a job is
started. It also renames the job prefix to be its own path so that the
JobManagerInterface can claim that as an object.

src/manager/root.rs

@@ -55,7 +55,7 @@ impl SteamOSManager {
             fan_control: FanControl::new(connection.clone()),
             wifi_debug_mode: WifiDebugMode::Off,
             should_trace: variant().await? == HardwareVariant::Galileo,
-            job_manager: JobManager::new(connection.clone()),
+            job_manager: JobManager::new(connection.clone()).await?,
             connection,
             channel,
         })