Add feature flag check for setting groupKey

In the 'shadow' new notification pipeline we cannot immediately set
the new override group key when the ranking is updated because the
old pipeline relies on checking whether the new/old override key is
different before updating. Therefore, only update the override group
key in the new pipeline once we're only using the new pipeline to render
notifications.

Note: this means that the shadow pipeline currently relies on the old
pipeline to set the override group keys (which may cause some
discrepancies since the timing is unreliable).

Test: manual + atest SystemUITests
Change-Id: I1afb303a86bf99d500163e108a9f141787267077