Support getSlicingConfig() API for 5G Slicing Configuration

- add getSlicingConfig() and add getSlicingConfigResponse() to handle
  the response
- add test case for getSlicingConfig() in RILTest

Bug: 178075247
Test: .
(1) run "atest TelephonyManagerTest#testGetSlicingConfig" for CTS result
[22/121] android.telephony.cts.TelephonyManagerTest#testGetSlicingConfig: PASSED (956ms)
(2) run "atest RILTest#testGetSlicingConfig" for RILTest result
[17/111] com.android.internal.telephony.RILTest#testGetSlicingConfig: PASSED (1ms)

Change-Id: Iee3c3b95957e56f0f4c58c5faa3a4a117e9615b8
Merged-In: Iee3c3b95957e56f0f4c58c5faa3a4a117e9615b8