Don't update footer if empty shade also visible

If the empty shade is visible, or the footer happens to be the first
visible view in the stack (notGoneIndex == 0), that means we're in a
transitory state so we don't need to update the position of the footer
in that run of the algorithm. This fixes the issue with the footer
jumping in weird ways when a notification is dismissed.

This change is flagged, but I tested it without the flag and it actually
also fixes an issue with the footer jumping when a notification appears
while the shade is open.

Fix: 332430751
Test: manually dismissed notification, posted notification and pressed
clear all with only one notif present
Flag: com.android.systemui.notifications_footer_view_refactor

Change-Id: I79e46622d782d93cab6fb98dfbbf8d11e6bf4b73