floss: Scanner: Remove existing/pending MSFT handles on start/stop_scan

This patch fixes the following potential issues:
* If start_scan() is called with the same scanner_id twice, all existing
  monitor handles would leak.
* If an MSFT command is pending but the scanner_id is unregistered and
  then a new scanner is registered with the same ID, then the handle
  would be wrongly attached to the new one.

Since the above issues are resolved in DBus handler layer, adjust the
logging levels for some affected code path.

Bug: 346656627
Tag: #floss
Test: mmm packages/modules/Bluetooth
Test: bluetooth_AdapterAdvMonitor.all_floss on
      WCN3991(qca quirk), RTL8852(rtl quirk), and WCN6856(no quirk)
Test: toggle NearbyShare and FastPair settings, start normal pair,
      verify the correct MSFT/Ext Scan parameter on WCN3991 and RTL8852
Test: verify normal pairing on RTL8822(no MSFT support).
Flag: EXEMPT, Floss-only changes
Change-Id: I9f283e056efed43c69a59ec4f32af0ee8260e802