floss: Scanner.StartScan: Use Passive scan by default

Currently ChromeOS doesn't have a feature requires Active LE scan.
Set default to Passive as a workaround for the FW issue on Trogdor,
while still allow the user to set Active in the future.

Bug: 290300475
Tag: #floss
Test: mmm packages/modules/Bluetooth
Test: Manual: Pairing, FP, NS sender/receiver, PhoneHub, and SmartLock
      on Cherry/Tomato, Guybrush/Nipperkin, Nami/Sona, and Trogdor
Test: bluetooth_AdapterAdvMonitor.advmon_hci_events_filtered_tests.floss
      on Cherry/Tomato, Guybrush/Nipperkin, and Trogdor
Test: bluetooth_AdapterLEHealth.all_floss on Nami/Sona
Test: bluetooth_AdapterQuickHealth.AVL.floss on Cherry/Tomato
Flag: EXEMPT, Floss-only changes
Change-Id: Ic528048099d060fbb8ef093c2ff3c556f04958b4