Default screen context permissions for NIU Actions

This updates the checks for ASSIST_STRUCTURE_ENABLED and ASSIST_SCREENSHOT_ENABLED to return 1 if the settings do not exist. This matches the logic in the Settings app iself, which has the toggle boxes on by default but does not actually initialize those values until they are toggled for the first time.

Bug: 187868947
Test: Manual (tested on Pixel 3A)
Change-Id: I4ef067dcf21b40eea71199e057d7a00d1d8fb2b3