Category message is sufficient for message notif

If a notification is marked with category message, then
it is sufficient enough to be deemed a "message" notification.
However, to be considered an important message, we still
check if the message is from the default messaging app and has
category = message.

Change-Id: I4f2b502634b805919bdf8b82e3bdf475c0992bdd
Fixes:76019310
Test: AttentionManagementVerifierActivity
Test: atest services/tests/uiservicestests/src/com/android/server/notification/NotificationComparatorTest.java