Stop leaking Notifications when they change groupKeys.

If a Notification changes groups (either from one group to
another, or gets a group key for the first time, which takes
it out of its implicit singleton group) it should be removed
from the old group in NotificationGroupManager's mGroupMap
and re-inserted under the new one.

Unfortunately we were passing the *new* notification in the
oldNotification argument, so we would always attempt (and
fail) to remove it from the *new* group, leaving a strong
reference to the Notification inside mGroupMap under an
obsolete group key.

Bug: 26561365
Change-Id: Ie5cdceb4f843dbd363652e00fbc0f3ac6f6ef247