Improve MediaButtonReceiverHolder toString()

This exposes the MBR's component name and session package name, which
will be quite helpful for the media_session dumpsys.

Bug: 205124386
Bug: 244312001
Test: adb shell dumpsys media_session
Change-Id: I48a6df78e8642e8ed8334fdfc75b1a6a52530e57

services/core/java/com/android/server/media/MediaButtonReceiverHolder.java

@@ -246,10 +246,13 @@ final class MediaButtonReceiverHolder {
 
     @Override
     public String toString() {
-        if (mPendingIntent != null) {
-            return "MBR {pi=" + mPendingIntent + ", type=" + mComponentType + "}";
-        }
-        return "Restored MBR {component=" + mComponentName + ", type=" + mComponentType + "}";
+        StringBuilder sb = new StringBuilder();
+        sb.append("MBR {pi=").append(mPendingIntent);
+        sb.append(", componentName=").append(mComponentName);
+        sb.append(", type=").append(mComponentType);
+        sb.append(", pkg=").append(mPackageName);
+        sb.append("}");
+        return sb.toString();
     }
 
     /**