Optimize and clean up BaseData

- Convert it to a record
- Improve encapsulation
- Finalize some fields
- Use stream() for more elegant code

Bug: 394958283
Test: atest BaseDataTest
Test: atest BassClientStateMachineTest
Flag: EXEMPT refactor no-op
Change-Id: I68d5155853b4484b5127b202da922129a167eff1