[3/N] Copy OomAdjuster compute methods to individual impls

The Cl moves the high level OomAdjuster policy methods to both
impls without changing any lines of code. This is purely in preperation
of the next CL that will trim down the compute methods to only what is
needed for their respective impls.

Also, removes the abstract `getInitial.*` methods from the base
OomAdjuster because the implementations of those methods were just used
by each impl to configure the `compute.*` methods and can be privately
referenced.

Flag: EXEMPT pure refactor
Test: atest MockingOomAdjusterTests
Test: builds and flashes
Bug: 306731120
Change-Id: Ia14d3260ef06e645f56ab7dcec286a30018b50d7