Fix NPE for system routing on MediaRouter2Manager-based InfoMediaManager

We need the package name of the manager/router owner even if we just
need to do system routing. Passing a null package name to
InfoMediaManager means the caller is only interested in system routing.

Test: adb shell device_config put media_solutions com.android.settingslib.media.flags.use_media_router2_for_info_media_manager true && adb reboot. Then test manually.
Bug: 316580657
Flag: NA
Change-Id: Ia09aa59a387dbe2f516ad9a9a40d2193121f4bf3