Moving setIsRootNamespace in notification one level higher

setIsRootNamespace was used to avoiding conflicts when we look for view by id but custom notifications layout happens to have view with the same id (b/13421971).
Unfortunately it also breaks keyboard navigation - once user enters notification with keyboard, they can't leave it.

The solution is to override focusSearch method but that can't be done on root view (id status_bar_latest_event_content) because it's coming from framework.
That's why setIsRootNamespace is called on view one level higher - NotificationContentView and then we override focusSearch.

Bug: 285554136
Test: PlatformScenarioTests:TabingThroughSplitShade
Change-Id: I8a6f0bc9012df8f55eb1ad1cb4c3d44af537b18c