am: Move OomAdjuster.initSettings() logic to ActivityManagerService

This CL moves OomAdjuster's initialization logic into
ActivityManagerService.setupServicePrewarmingOnUserSwitch(). This
includes transferring the ACTION_USER_SWITCHED receiver registration.

Bug: 441408003
Test: m services.core
Test: atest MockingOomAdjusterTests OomAdjusterTests
Test: atest FrameworksServicesTestsRavenwood_ProcessStateController
Flag: EXEMPT PURE_REFACTOR

Change-Id: I1d642980d8cbed145ab1b1b23eff1f466f65c84d