Use generated RemoteViews preview for widget picker if available

WidgetCell will use the generated RemoteViews preview to display
previews in the widget picker. It will fallback to current preview
methods if not available.

Introduces WidgetManagerHelper.loadGeneratedPreviews as a utility
function. WidgetManagerHelper is passed into the constructor of
WidgetItem to avoid calling
context.getSystemService(AppWidgetManager.class) for each widget.

Bug: 308041327
Test: atest Launcher3Tests:GeneratedPreviewTest
Flag: ACONFIG com.android.launcher3.enable_generated_previews DEVELOPMENT
Change-Id: I37429057cda83a5321884ace2537038e050b9a58