NIU Actions: Add assistant error dialogs

The NIU Actions buttons send intents to the user's default assistant. If the user clicks one of the buttons without an appropriate default assistant set, a dialog will appear to explain the situation and redirect the user to Settings.

The existing privacy confirmation dialog code has been refactored into a reusable dialog, which is used here.

Bug: 192505939
Test: Manual on local Pixel 3A (unit tests to be added in b/192406446)
Test: m -j RunLauncherGoGoogleRoboTests
Change-Id: I7b43e008aa74805b166c09936fa938a5834c8460