Fix filtering logic for stopped packages

System apps can also be in a stopped state, per b/249514169. As such,
while checking if a package is stopped for a user, skip checking if its
not a system package.

Bug: 249514169
Test: atest CtsContentTestCases:SystemPackageDefaultStoppedStateTest
Change-Id: Ib64f3b62cb8a3e1d580d55bfdcd904e52103d394