Make biometric authentication more robust

[chenxiaolong]

• Hide the entire window contents instead of just the fragment contents when unauthenticated.
• Make invisible views unfocusable and drop all input events when unauthenticated.
• Set FLAG_SECURE when the window loses focus to avoid having the screenshot show in the recent apps switcher.
• Move the authentication enforcement to the base activity instead of the base fragment since the activity is what's directly attached to the window.
• Use the top-level activity in each task to enforce authentication restrictions to allow predictive back gestures to continue to work.