Move abstraction of activity priority calculation

This converts ComputeOomAdjCallback interface to exported flags.
So it is easier to separate the dependency, add new conditions,
and handle state consistently in the future.

Bug: 302575389
Bug: 200769420
Flag: EXEMPT refactor
Test: WindowProcessControllerTests#testComputeOomAdjFromActivities
      MockingOomAdjusterTests
Change-Id: I1f43ec2c6701ef1240ce483f8a2d14972bc2a01c