Support getSlicingConfig() API for 5G Slicing Configuration

- add getSlicingConfig() API in TelephonyManager and ITelephony
- add Parcelable implmentations for RouteSelectionDescriptor,
  SlicingConfig, and UrspRule
- remove @hide and @SystemApi for TrafficDescriptor and
  NetworkSliceInfo to make them public

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: I0addc2b22c11b13759e890d88f51490993fb72e1