Floss: LE Scan API change: optional ScanFilter and u8 in ScanFilter

* Filter should be optional in StartScan to allow clients to do general
  LE scan.
* ScanFilter fields should be u8 to match the internal type in MSFT HCI
  extension.

Bug: 217274432
Tag: #floss
Test: Build Floss on Chrome OS and Linux

Change-Id: Iad9aa2c556fe94099f56462de81e6c027ea0145d