Animate top padding of notifications on status view height change.

Note that `requestScrollerTopPaddingUpdate(true);` doesn't work because
the topPadding doesn't update until later, through the
`ShadeLayoutChangeListener.onLayoutChange`.

Fix: 293564374
Test: on Keyguard turn on/off flashlight via shortcut
Test: onKeyguardStatusViewHeightChange_animatesNextTopPaddingChangeForNSSL()
Change-Id: Id3cee8616df497f0957c25cd1ba56a4dbfbfbc0e