am: Refactor ActiveUids state observation

ActiveUids now uses a generic `Observer` interface to notify about
UID active/inactive state changes. ActivityManagerService sets its
ActivityTaskManagerInternal instance as this observer.

This change decouples ActiveUids from a direct dependency on
ActivityManagerService and its internal components, improving
modularity and clarity of the notification flow.

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

Change-Id: I0ae990a9d459dd79306cd5273ba54460561b9a95