Optimize and clean up AppScanStats.java

- Declare some fields as `final`
- Improve encapsulation of some functions
- Use `requireNonNullElseGet()`
- Introduce `createBaseScanEvent()` to reuse code
= Use `values.()` over `keySet()` when value is required

Bug: 394958283
Test: atest AppScanStatsTest
Flag: Exempt refactor no-op
Change-Id: I71fc2ab20ad4aa2abd6eb7e4c4c7015c235bb7ce