am: Make maybeSetProcessFollowUpUpdateLocked use internal class

This CL makes the `OomAdjuster.maybeSetProcessFollowUpUpdateLocked()` to
operate directly on `ProcessRecordInternal`. To achieve that, it moves
`mKilled` field from `ProcessRecord` to `ProcessRecordInternal`.

Bug: 425766486
Test: m services.core
Test: atest MockingOomAdjusterTests OomAdjusterTests
Test: atest FrameworksServicesTestsRavenwood_ProcessStateController
Flag: EXEMPT pure refactor
Change-Id: I734e23fd7fbf810a964a3c2d75b3905984a147b9