am: Remove CacheOomRanker

Thic CL removes the `CacheOomRanker` class and its associated test file,
and cleans up all references to `CacheOomRanker` within
`ActivityManagerService` and `OomAdjuster`.

Bug: 439720579
Test: m services.core
Test: atest MockingOomAdjusterTests OomAdjusterTests
Test: atest FrameworksServicesTestsRavenwood_ProcessStateController
Flag: EXEMPT PURE_REFACTOR

Change-Id: If9c4a78f97b825bbc22caac53837ef4ac5143ce1