Extract `GattServiceBinder` to its own class

- One level less indentation = shorter lines = shorter code
- Add getters
- Some visibility is expanded for this division

Bug: 401438096
Test: atest GattServiceTest
Test: atest GattServiceBinderTest
Flag: EXEMPT refactor no-op
Change-Id: I096308b37f474f3504df3164228c63fd554a0d18