Add tap-to-add button to widget picker

This change introduces an "Add" button that appears when a widget
preview is clicked in the widget picker. This button disappears when the
preview is clicked again, or another preview is clicked. When the button
is pressed, it adds that widget to the picker. The add button is
available in the app-specific widget sheet as well.

Bug: 323886237
Test: Manual
Flag: ACONFIG com.android.launcher3.enable_widget_tap_to_add DEVELOPMENT

Change-Id: I86a8a4c22119960c54a885fd2efeb91916b4f9a0