system/ui: add require_full_screen to WifiManagerUI

[deanlee]

Adds a require_full_screen property to the WifiManagerUI class that allows callers to detect when the WiFi UI is displaying a forget confirm dialog or password prompt that should block other UI elements from rendering.

Testing:

Verified that the system/ui/updater.py screen properly handles WiFi password prompts by only rendering the password UI when active, avoiding UI element conflicts like before that BACK button overlapped displayed on top of keyboard: