Add API to disable snapshotting of activities

Test: runtest frameworks-services -c
com.android.server.wm.TaskSnapshotControllerTest
Test: Launch DisableScreenshotsActivity, go to recents, make sure
content is white.
Bug: 31339431

Change-Id: I329925d2fca389e561da3389a67fe888b5bb1033
12 files changed