Floss: Modify IScannerCallback

IScannerCallback needs a dedicated callback for clients to listen to
when advertisements are in the beginning of RSSI-range instead of every
advertisements received during the range. So this modifies
IScannerCallback methods:
* Add OnAdvertisementFound: beginning of adv in RSSI range
* Add plan to rename OnScanResultLost to OnAdvertisementLost for
  symmetry. Will be in future change since this needs to be coordinated
  with Chrome change.
* Redefine OnScanResult to be explicit that this is triggered on every
  advertisement detected if satisfies the filter and RSSI range.

Bug: 269343922
Tag: #floss
Test: Build Floss on Linux and Chrome OS, run with chrome-btclient.

Change-Id: I0a0c683f48586deb4d2227f1cb011c5ee09e95e6