add capability for TelephonyManager.getNetworkSlicingConfiguration()

- add comment to specify HAL interface getSlicingConfig() which is used
  by TelephonyManager.getNetworkSlicingConfiguration() is in the reduced
  feature set.
- allow REQUEST_NOT_SUPPORTED for HAL interface getSlicingConfig() and
  update related VTS test case.

Bug: 181713905
Test: run "atest VtsHalRadioV1_6TargetTest" and check the result for
      getSlicingConfig is PASSED
[3/17] PerInstance/RadioHidlTest_v1_6#getSlicingConfig/0_slot1: PASSED (9ms)

Change-Id: Ia318b8225e8b0e772a85bb9909f2e0114e0ae6e3