Exit split-screen when primary stack moved behind a fullscreen stack.

Activities can request their task be moved to the back of the stack,
which subsequently causes the stack to move as well. When the stack
is a split screen primary stack, this causes an ordering issue where
the secondary will not move back while a fullscreen stack has moved
in front of the primary. This leads to further ordering issues once
the front secondary is dismissed as other stacks are also in the
secondary windowing mode.

To address this issue, we exit split-screen mode when the primary
split-screen stack is moved back.

Test: atest ActivityStackTests#testPrimarySplitScreenToFullscreenWhenMovedToBack
Change-Id: Ic0597831e046a254b3cba216e1cb2fb11191f2c6
Fixes: 69662547