Add onSupportedTetheringType callback

This new callback could tell caller Tethering is supported for what tethering
types.

Bug: 184996041
Test: atest TetheringTests
      atest EthernetTetheringTest
CTS-Coverage-Bug: 223340235

Change-Id: Ib80ed8d7f73f4a098b8965db186d24d8cf1884d3
6 files changed