Floss: support scan settings parameter for StartScan

This patch updates the StartScan method signature to accept a scan
settings parameter. If provided, the scan settings will be passed to the
native Bluetooth stack. Otherwise, default scan settings will be used.

Bug: 304681854
Bug: 300216447
Tag: #floss
Test: ./build.py --target test
Test: m -j
Change-Id: Ie262e34ed364134e9baacaf45885d9e275581fd0