psc: Allow ServiceRecordInternal to access ConnectionRecordInternal

This change introduces two new methods to `ServiceRecordInternal`:
`getConnectionsSize()` and `getConnectionAt()`. This allows
`OomAdjuster` to iterate through the connections of a service without
needing to know the concrete implementation of `ServiceRecord`.

Bug: 425766486
Test: m services.core
Test: atest MockingOomAdjusterTests OomAdjusterTests
Test: atest FrameworksServicesTestsRavenwood_ProcessStateController
Flag: EXEMPT pure refactor
Change-Id: I3156154b3c88af70b53b9656e3407e232cf84d20