ProcessStateRecord: Publicize and document API

Publicized methods in ProcessStateRecord and added Javadoc for
non-trivial public APIs. This is the preparation for moving the class
into psc package.

Bug: 425766486
Test: m services.core
Test: atest MockingOomAdjusterTests OomAdjusterTests
Test: atest FrameworksServicesTestsRavenwood_ProcessStateController
Test: prebuilts/checkstyle/checkstyle.py --file ProcessStateRecord.java
Flag: EXEMPT pure refactor

Change-Id: Ia6aeeec93725a1cf49d4e225ef122a3cf51a4428