Allow at most one pinned stack task.

This changelist enforces only one task may be present in the pinned
stack at a time. If a task is already persent, the existing task is
moved to the fullscreen stack.

Fixes: 36844394
Test: cts/hostsidetests/services/activityandwindowmanager/util/run-test CtsServicesHostTestCases android.server.cts.ActivityManagerPinnedStackTests#testPipMovedToFullscreenStack
Test: bit FrameworksServicesTests:com.android.server.am.ActivityStackSupervisorTests#testDisallowMultipleTasksInPinnedStack

Change-Id: Iaf0fbda6df835d93738fdf6f7f3a8c5956c2b262