am: Move CachedAppOptimizer to ActivityManagerService

This CL moves the field of `CachedAppOptimizer` from `OomAdjuster` to
`ActivityManagerService`, and exposes it via `getCachedAppOptimizer()`
for direct access.

Bug: 441178013
Test: m services.core
Test: atest MockingOomAdjusterTests OomAdjusterTests
Test: atest FrameworksServicesTestsRavenwood_ProcessStateController
Test: atest BroadcastQueueTest ServiceBindingOomAdjPolicyTest
Test: atest ServiceTimeoutTest
Flag: EXEMPT PURE_REFACTOR

Change-Id: Ic890a0ff8126f305e961e6a1564a984cc9482c52