Scroll to show WidgetCell when it is tapped.

Scroll to show WidgetCell when it is tapped in a widget sheet.
Otherwise, the add button may show/hide without the user seeing
it if the bottom is clipped.

Bug: 329861721
Test: manual- tap WidgetCell when top or bottom is scrolled out of view
Flag: ACONFIG com.android.launcher3.enable_widget_tap_to_add TEAMFOOD

Change-Id: Ie21730c193e845cb1c1fa447b7c0a7e719984a8f