Add apex-system-service to apex.test.

The test apex is being used for ApexManagerTest, and the addition to
manifest is used to check parsing of apex-system-service.

Bug: 192880996
Test: atest com.android.server.pm.ApexManagerTest#testGetApexSystemServices
Ignore-AOSP-First: changes depend on new features in PackageManager
Change-Id: I42c53216b71a2b6c6dd86c6370d5489052bc905b
2 files changed