Move PSC async Activity update methods to a helper class

The new ActivityStateAsyncUpdater is purpose built for Window Manager to
update ProcessStateController about Activity state changes that can
affect importance computations.

Flag: com.android.server.am.push_activity_state_to_oomadjuster
Bug: 401350380
Test: atest MockingOomAdjusterTests
Change-Id: Iaa1faac9e82f67e23a7448c428a4c67ff2d9b98e