Fix CarNotificationListener test

The mRankingMap used in the tests had overrideGroupKey value as null.
setOverrideGroupKey will not be called if the
new mStatusBarNotification have the same overrideGroupKey value.

Bug: 231736086
Test: atest com.android.car.notification.CarNotificationListenerTest
Change-Id: I861f1b60f52c546870ac6bad1c6e9c6074cc8535
1 file changed