De-flake testFakeControlTarget_overrideVisibilityReceivedByWindows

This CL let app2 be the focused window to make sure the control target
won't be changed unexpectedly while we are waiting for the idle of
WindowAnimator.

Fix: 328585829
Test: atest InsetsPolicyTest
Change-Id: I56efd7e39044a0c7450e5bda79d6e09ec7f13b33