Floss: Always set scan parameters for LE scan

If the LE scan parameters are not specified, libbluetooth will default
to 3s for both the scan window and interval, which means 100% duty scan
cycle. This might leave the controller with no chance to do other
activities, e.g. connecting to LE devices.

This CL makes sure that we always set the scan parameters before every
LE scan start attempt.

Bug: 298602709
Tag: #floss
Test: m -j
Test: Activate FP background scan, power cycle BT adapter, then
      reconnect any HID LE device. Verify reconnection is immediate.
Change-Id: I4d88f1db1dcfb9c6ab92c2d667798398a0aa8277