Revert "Cleanup flag "floating_menu_narrow_target...""

This reverts commit dd281f6b.

Reason for revert: To fix http://b/376473165, we need to re-add the content observer code. Due to the complexity of this change, it's easier to revert this change as a foundation for the fix for http://b/376473165. See http://b/376473165#comment18 for more details.

Bug: b/376473165
Change-Id: Iafa29233082584141e4604457ba12ac626a8a0bd