Return null from getWidgetPreview instead of throwing error

If the provider is not found or the caller does not have permissions to
access the preview, return null from getWidgetPreview instead of
throwing IllegalArgumentException. This fixes certain launcher tests
that do not run with BIND_APPWIDGET permission.

Bug: 308041327
Test: Launcher3Tests:TaplOpenCloseAllAppsTest
Change-Id: Ie1617e2ef26ac3b4e7ec4a1dd0d3a3a71e50cd77