psc: Refactor ServiceRecord host process access

Introduce `ServiceRecordInternal#getHostProcess` and
`ServiceRecordInternal#getIsolationHostProcess` to abstract how the
process hosting a service is accessed. Update `OomAdjuster` and
`OomAdjusterImpl` to use this new method instead of directly
accessing `isolationHostProc` and `app` fields.

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

Change-Id: I7bccd1ce70b8d92aac5be7c5da1e620127ede7f3