Address a TODO in TetheringConfiguration to use a dependency object
instead of static mocking on DeviceConfig.
This avoids TetheringConfiguration dependencies on internal
implementation of DeviceConfigUtils, unblocking changes in
DeviceConfigUtils.
Bug: 279108992
Test: atest TetheringTests
Change-Id: I252eaadff85fa47b894e989b4f2527b00c5dca56