Fix broken hibernation test

ag/14203210 made isAppHibernationEnabled use a cached value rather than
query device config every time. This happened to break a test that
relied on mocking the DeviceConfig value. We update the static value
directly to ensure the feature is enabled in the test.

Bug: 185730191
Test: atest PackageManagerServiceHibernationTests
Change-Id: Icf7d5bedba69cc5af7df2d38bf5088cb08b5cfb6