Keep Add and Done visible as secondary buttons when a widget is selected

In glanceable hub edit mode, show Add and Done buttons as
primary button when nothing is selected. Show them as outline
button when a widget selected or when reordering.

Add basic fade animation to the toolbar buttons.

Bug: b/341752442
Fix: b/341752442
Test: manually tap, unselect, reorder widgets
Flag: com.android.systemui.communal_hub
Change-Id: I933cb1b5587b9909c6034a0d58bbd599cd446b8d