Fix notif content when child updated to top level

Since children can be updated to not be in groups and become a top level
notification, we need to check and ensure that we inflate the content of
any top level notifications that don't have content.

Bug: 154227833
Test: repro in bug
Test: atest DynamicChildBindControllerTest
Change-Id: I83120691e20f63ee4f3d440d84c005cdfe84a719