Add ProcessStateController batching API for Activity state changes

Activity related events (screen state transtions, top activity changes,
task layer reordering) can result in many state changes being pushed to
ProcessStateController. This CL adds a BatchSession object that will
group all the state changes triggered while the session is active to one
Handler thread message.

Flag: com.android.server.am.push_activity_state_to_oomadjuster
Bug: 401350380
Fixes: 399680824
Test: atest MockingOomAdjusterTests
Test: atest ProcessStateControllerTest
Change-Id: I61a6474397bc06b8f9be07c6f2b44d624c9a1cb6