Position stack at top when always on top flag is set

When always on top is set, the position of the stack must be
changed in ActivityDisplay and in DisplayContent. This CL
introduces this logic in ActivityStack.setAlwaysOnTop().

Bug: 69370884
Test: go/wm-smoke
Test: atest DisplayContentTests
Test: atest WindowContainerTests
Change-Id: Ie7efe175175a4db209a6b0f3476d9dfc27432df5