Make LaunchingTaskPositioner stateless.

The initial design cached calculations based on the available bounds.
This required the positioner to be involved in stack lifecycle events
and tied to a particular stack.

This changelist removes these dependencies and makes the
ActivityStackSupervisor the owner.

Bug: 64144308
Test: bit FrameworksServicesTests:com.android.server.am.LaunchBoundsTests
Change-Id: I4e2df4398b0818cec71d3d90fe409012191406c0