Add functionality for new smart action

When user clicks on smart action MARK_CONVERSATION_AS_PRIORITY, open the
conversation notification's notification guts with "Priority"
pre-selected. User will still need to click "apply" for the new priority
to be applied.

Test: manual, atest NotificationConversationInfoTest
Bug: 163614153
Change-Id: Ic22261f6b81274e42f20f1748e1465a03dee8144