am: Add OomAdjuster.Callback interface

This CL adds the OomAdjuster.Callback interface to decouple OomAdjuster
from direct dependencies on the am package. Some of the
CachedAppOptimizer functionality is now wrapped within an implementation
of this new interface.

Bug: 441178013
Test: m services.core
Test: atest MockingOomAdjusterTests OomAdjusterTests
Test: atest FrameworksServicesTestsRavenwood_ProcessStateController
Test: atest ProcessStateControllerTest
Flag: EXEMPT PURE_REFACTOR

Change-Id: I7750b785f5a516d0a31415be2e829302745d7357