Make Tethering public API for DO/Carrier apps

Make a limited subset of Tethering APIs public for DO/Carrier apps
to start Wifi tethering with a custom SoftApConfiguration.

The following APIs are needed to be moved to public
- TetheringManager
    - startTethering
    - stopTethering
    - registerTetheringEventCallback
    - unregisterTetheringEventCallback
- TetheringManager.TetheringRequest
    - getSoftApConfiguration
- TetheringManager.TetheringRequest.Builder
    - setSoftApConfiguration
- TetheringInterface
- TetheringEventCallback
    - onTetheredInterfacesChanged

Checks for DO/Carrier apps will be added in a later CL.

API-Coverage-Bug: 374400379

Bug: 216524590
Test: atest TetheringTest TetheringManagerTest
Change-Id: I9fb420ec6cebea17d8f3bc40a0ae8735074d9eb0
5 files changed