Don't allow stacks above pinned stack.

Previously, stacks would be allowed above the pinned stack if the
pinned stack was not visible. We however do not correct the stack
position when the pinned stack later becomes visible. This can lead
to a scenario where the pinned stack has become visible yet is below
other stacks. When a stack is positioned in this state, an
IllegalStateException is thrown to indicate the invalid pinned stack
position.

Since the pinned stack should only be present and populated when
there is a pip activity, we can instead use its presence as an
indicator other stacks should be underneath it. This avoids placing a
stack above the pinned stack during transitional states where the
stack children might not be visible, therefore preventing the above
exception from occurring.

Fixes: 36669386
Test: bit FrameworksServicesTests:com.android.server.wm.DisplayContentTests#testPinnedStackLocation
Change-Id: I67a58806e83b11f0b135c12441bb0a9bec58eee7