Fix AccessibilityShortcutChooserActivityTest for small screens.

This test used to use UiAutomation to find and click the floating dialog
buttons because UiDevice is unable to find system windows. This didn't
work on small screens that had to scroll first to show a specific
button.

Rather than adding in scrolling, which itself is flaky and requires
waiting, it's much simpler to just directly grab and invoke the dialog
button click actions because the dialog is available to the test
activity.

Also adds a note warning the developer about how this test intercepts
and creates a custom A11yManager, which can cause confusing UI
inspection behavior.

Bug: 315929374
Test: AccessibilityShortcutChooserActivityTest 50 iterations
Change-Id: I15437c91bf4e17de466fe34bd45ffcae5478baab