Flag to bypass shortcut req for "convo" notifs

Enabling this flag in

  Settings > System > Developer options > Feature flags

will bypass the ShortcutId requirement for MessagingStyle
notifications to be classified as "conversations". This exists in
order to test this style of notifications on real apps, before the API
exists for them opt-in for real.

Test: manual
Bug: 147591608
Change-Id: Ifaba2891f2101f1151053290c57f5249c13a46c1