Add tests for 5G Slicing feature APIs

- test the contructors/setters/getters for NetworkSliceInfo,
  NetworkSlicingConfig, RouteSelectionDescriptor and UrspRule

Bug: 184396511
Bug: 184396554
Bug: 184396555
Bug: 184396594
Bug: 184396595
Test: atest NetworkSliceInfoTest
      atest NetworkSlicingConfigTest
      atest RouteSelectionDescriptorTest
      atest UrspRuleTest
Change-Id: I465ab051614978c735acd7a76f07f4ede35c45eb