Fixed the behavior of group conversations when targeting P

Fixes an issue where the group conversation heuristic was still
applied even when the app is targeting P. We're now following
the isGroupConversation completely and only do the migration
for apps targeting < P.

Change-Id: I471d58e8a8f5e6270f0dcce8691d08a2bdd1c582
Fixes: 78450835
Test: add messaging notification, observe correct display