Add support for generated previews to People Space widget

This change updates PeopleSpaceWidgetManager to use
AppWidgetManager.setWidgetPreview to provide widget picker previews.
PeopleSpaceWidgetManager will set the previews when the user is first
unlocked.

Bug: 308041327
Test: PeopleSpaceWidgetManagerTest#testUpdateGeneratedPreview_*
Flag: ACONFIG android.appwidget.generated_previews TEAMFOOD

Change-Id: Ie838d014c624d1a34c4819a579fa32fdfefb5880