Mock ActivityStateAsyncUpdater.startBatchSession()

When Flags.pushActivityStateToOomAdjuster() is turned on, some spots in
WindowManager are expecting the BatchSession object to exist.

Flag: com.android.server.am.push_activity_state_to_oomadjuster
Bug: 434856878
Test: atest com.android.server.wm.ActivityStarterTests
Change-Id: I31ac701644a3374002b728e8f9274c2b6b7db75a