Handle existed source first, then handle QR code.

If the source in the QR code already exists (either streaming or paused) on a device, we don't need to start a targeted scan for that source.

Flag: com.android.settingslib.flags.audio_stream_scan_with_filter
Test: atest
Bug: 395978182
Change-Id: Iccedaa82f95a609a99cc0a9275c1c57f90329dd7