Remove no-op TetheringEventCallback default function

From API guideline: for platform code and libraries targeting Java 8 or
later, should prefer interfaces in cases where the default implementation
is stateless. Remove no-op default funnction call in
HotspotControllerImpl, then change TetheringEventCallback from abstruct
class to interface with default methods.

Bug: 151243337
Test: m, atest
Change-Id: I694f9110cffb7be8e4fd99bf6ac29e8a47131326