Move oom adjust related code to OomAdjuster

This is part 2 of moving process management related
code out of ActivityManagerService

Move all the compute/update/applyOomAdjust methods,
leaving some shims behind for convenience.

Bug: 111369273
Test: Build and boot
      atest CtsAppTestCases
      atest ActivityManagerInternalTest
      atest ActivityManagerServiceTest
Change-Id: Ie8d203bbdff5a8a7b86a80b368f6125b6a23c949