Separate out test for bound widget package from setup

- setup fails when AppWidgetManager is disabled
- therefore, make a seperate test for bound widget package and skip the
test when app widget manager is disabled

Bug: 137217082
Test: run atest com.android.server.usage.AppStandbyControllerTests
ASSUMPTION_FAILED for testBoundWidgetPackageExempt and PASS for the rest

Change-Id: If1575d297d395bbd1e9dfb8b4729998a4dc036e0