Reset selected Notification importance after closing the guts

The selected importance state was never resetted which resulted in some inconsistent behaviour after binding a NotificationInfo view again. It is fixed by resetting it upon closing the guts.

Remove unused fields from the PartialConversationInfo.
Add a new test for NotificationGuts to check if the close callback is
called.

Test: atest NotificationInfoTest
Test: atest NotificationConversationInfoTest
Test: atest PartialConversationInfoTest
Test: atest NotificationGutsTest
Test: manual

Fixes: 241915559

Change-Id: Ibfcf88107e6b8548d7d98138925be1671c64a176