Ensure user can't block bg usage for protected pkg

Also ensure that standby buckets are refreshed after changing protected
packages, so that the apps are put into exempt bucket promptly.

Bug: 326031059
Test: manual
Test: atest CtsDevicePolicyTestCases:android.devicepolicy.cts.UserControlDisabledPackagesTest
Change-Id: I5951b7c3e8fbc9ec81e8df1cd6e0d017bb4b452f