floss: Make sure scan mode is off when powered off

This avoids a race when Floss is powered off and the system immediately
enters suspend.

Bug: 361510982
Tag: #floss
Test: mmm packages/modules/Bluetooth
Test: bluetooth_AdapterSRHealth.sr_while_powered_off.floss
Flag: EXEMPT, Floss-only changes
Change-Id: I6e1ac6a5778440b3a13a9e561bbe4b0df422084f