Refactor tethering utility methods

This is no-op refactoring to extract tethering utility methods
to a standalone file, and split setup and clean up process
to allow new test cases can perform custom verifications.

Also, this CL makes connectToWifi returns NetworkHandle for later use.

Test: m connectivity_multi_devices_snippet && \
      atest CtsConnectivityMultiDevicesTestCases:ConnectivityMultiDevicesTest
Bug: 335368434
Change-Id: Ib0df28107fd09b573db1e51ff624d0f027bda23d
4 files changed