Adjust the bottom padding padding on the left pane in picker

Right now, with 24dp bottom padding, the last item is partially visible
behind that padding. And makes the touch target for last item small.
This is more visible when show all button ends up there.

Bug: 356127021
Flag: com.android.launcher3.enable_tiered_widgets_by_default_in_picker
Test: Unit tests
Change-Id: If2743f73bec0ea244fef7eb1fa3efc903876914b