Don't prepare transition if token is not visible

This led to an issue that atoken was added to opening apps,
regardless of its requested visibility state, thus making apps
visible that should not be visible at all, because once it's
added to opening apps, it will become visible when the app
transition is being executed.

Test: cts/hostsidetests/services/activityandwindowmanager/util/run-test
android.server.cts.KeyguardTests#testUnoccludeRotationChange

Bug: 32771855
Change-Id: I375db25658ff95a6c6cc8e204486a1e73d2b2b88