BigPictureStyle: set unused big picture extra to null

ag/24426105 changed BigPictureStyle so that whichever of EXTRA_PICTURE
or EXTRA_PICTURE_ICON wasn't set to the big picture would be *removed*
instead of set to null.

This was an app-visible behavior change, and turned out to break one of
our own tests, b/315128763. Since it has the potential to affect apps
that interact with notifications, I'm modifying it to preserve the old
null value behavior, while leaving the other bitmap expiration work in
that change alone.

Bug: 290381858
Bug: 315128763
Test: atest NotificationTest
Test: atest NotificationManagerServiceTest
Flag: NA
Change-Id: Ifef48da0ba6c0314dffef9ac04e5018bfb85b91b