Improve equals method of NetworkScanRequest

Equals method is improved, following best practice:
- Make sure the object always equals to itself
- Add nullability annotation to non-primary fields
- Use Objects.equals to be null-tolerant
- Add UT cases to cover corner cases

This CL doesn't change API surface.

Bug: 287100905
Test: atest NetworkScanRequestTest
Change-Id: I662861c093856a04b560d6a0ee8cfd21557aee86