Do not let snapshot window control the bar

The snapshot window will sometime take over the system bar control as
the top fullscreen opaque window. To avoid the window take the control
if the window is not focusable to avoid the bar flickering issue when
the snapshot window has a different insets visibility with the current
controller.

Test: Launch sticky immersive app from recent. The bar hidden animation
      works correctly. No flickering.
Test: atest MultiDisplayPolicyTests
Test: ActivityStarterTests
Bug: 203017444
Change-Id: I3d7ba6b7d78c0508ddeccbf1d3408d723bd8e2c0