Disallow task snapshot starting window for intent != ACTION_MAIN

We don't want to show a task snapshot if the intent wasn't the
launcher intent. Likely the app will show something different, so
we shouldn't show a snapshot in this case.

Test: AppWindowContainerControllerTests
Test: Open app, make sure we get snapshot window
Test: Open Chrome, go home, Open chrome incognito from shortcut,
make sure no flash

Change-Id: Ib608ba8070ce09f418f1036248d81eebfa354128
Fixes: 35099602