Extract a new scanner pointer outside of gatt interface

See go/scan-manager-refactor for more details.

Bug: 313335632
Bug: 267361243
Bug: 327503826
Bug: 344711538
Flag: EXEMPT, mechanical refactor
Test: m com.android.btservices
Change-Id: I21803b7a4935360f0bbc48c20fecce1ddbfa507c