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
bac7f0085e