Optimize and clean up ScanController #2

- Reorganize variables
- Rename `stop()` to `cleanup()` for consistency
- ScanHelper/Scan Helper is no more

Bug: 394958283
Bug: 401837157
Test: atest ScanControllerTest
Flag: EXEMPT refactor no-op
Change-Id: I010b0877e30497be48000de62549330675b1a909