Refactor GattService for conciseness

- Prefer `AttributionSource source` over `AttributionSource attributionSource`
- Static import `toAnonymizedAddress` due to 63 usages

Bug: 394958283
Bug: 400833674
Test: atest GattServiceTest
Flag: Exempt refactor no-op
Change-Id: I8064dfa51c73d57af7809b17ce7abbd4fc2066a9