Move activity OOM adjustment computation to OomAdjuster

The logic for calculating the initial importance of processes with
activities (`computeOomAdjFromActivitiesIfNecessary`) has been
relocated. This functionality, previously in `ProcessStateRecord`,
is now within `OomAdjuster` via the new `OomAdjWindowCalculator`
inner class.

This change reduces the dependency of `ProcessStateRecord` on
`ProcessRecord` and centralizes the OOM adjustment logic within
`OomAdjuster` for better modularity.

Bug: 425766486
Test: atest MockingOomAdjusterTests
Flag: EXEMPT pure refactor

Change-Id: I6a540a50a7f826ca37218566f62afd29d52b93d8