add capability for TelephonyManager.getNetworkSlicingConfiguration()

- create CAPABILITY_SLICING_CONFIG_SUPPORTED in TelephonyManager
- mark up getNetworkSlicingConfiguration() with
  CAPABILITY_SLICING_CONFIG_SUPPORTED

Bug: 181713905
Test: atest RadioConfigResponseTest
Change-Id: I68399eee66e43a719598961cb1a0d00db6b4e25a