am: hide details of OomAdjObserver from OomAdjuster

This patch moves reportOomAdjMessageLocked() from OomAdjuster to
ActivityManagerService to hide certain AMS variables from OomAdjuster.

A future patch will abstract further this method by exposing it to
OomAdjuster through an interface or dependency class agnostics of AMS.

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

Change-Id: Iabf6039868c437b7bf189934297c87eaf3f1626b