Send NIU Actions Intents to default Assistant

This changes the recipient for NIU Actions from a package hard-coded in a config file to the currently selected default Assistant. The user can change this under Settings -> Default apps.

ImageActionUtils has also been updated to handle the case where its
Intents cannot be resolved, preventing a crash if the user sets the
default Assistant to something that cannot receive NIU Actions.

Bug: 186655099
Test: Manual (Pixel 3A, set default Assistant to Google Go and None)
Test: m -j RunLauncherGoGoogleRoboTests
Change-Id: I03b93597092575902d507615f32b63f4050c4c69