Check screen context permissions for NIU Actions

This checks for the ASSIST_STRUCTURE_ENABLED and ASSIST_SCREENSHOT_ENABLED permissions before sending NIU Actions Intents. If either permission is absent, the screenshot is not sent, and an error code is added to the Intent.

Bug: 182010359
Test: Manual (toggled permissions and clicked NIU Actions buttons on Wembley device)
Test: m -j RunLauncherGoGoogleRoboTests
Change-Id: Icf0db739f5010862cc2bac496f5410665625f3be