floss: check msft supported

The start_scan() and stop_scan() should check whether the MSFT
extension is supported before doing add/remove/enable operations.

Bug: 270629001
Tag: #floss
Test: ./build.py --target test
Test: Manual test on a ChromeOS device not supporting the MSFT
extension, e.g., octopus/blooglet
  localhost # btclient
  bluetooth> le-scan register-scanner
  bluetooth> le-scan start-scan 1
  bluetooth> le-scan stop-scan 1
The btadapterd should not crash. The btclient should not output
messages like
  main/shim/le_scanning_manager.cc - MsftAdvMonitorEnable: in shim layer

Change-Id: I41fca51eb6418332e03df052164893a3105cd7eb