Show persistent notification for page-agnostic mode

When device enters page-agnostic mode using 16KB developer
options, show notification to user using boot receiver and service.
On clicked on notification, show detailed instructions on how to
get back to production mode. Removing OEM carrier unlock allowed
condition.

Bug: 295035851
Bug: 338139755
Bug: 302600682
Test: m Settings && adb install -r $ANDROID_PRODUCT_OUT/system_ext/priv-app/Settings/Settings.apk
Change-Id: Ib7a57af4c6151d2a8da1ec94130532d10b1679aa