Fix KeyValueBackupTaskTest shadow errors

Replace IPackageManager with PackageManagerInternal to avoid shadowing
ActivityThread. This shadow is prone to unrelated framework changes breaking
our tests.

Bug: 130540953
Test: 1) atest RunBackupFrameworksServicesRoboTests
2) atest AppBackupUtilsTest

Change-Id: Iea95a37da3df9d3f0887ed5ab30c7007306415a1