Issue #7818: Use different target activities for launcher activity alias (fenix vs. fennec). (#7917)

* With this patch fenix build variants will launch HomeActivity directly from the launcher.
  This will get rid of the performance regression outlined in #7818.

* Fennec build variants will launch into MigrationDecisionActivity which will show the
  migration UI or launch normally into the app. This is faster than going through
  IntentReceiverActivity (measured and perceived).