[SP18.3] Adapt TestableNetworkStatsProviderCbBinder

This is a no-op refactoring to adapt new test provider callback.
Also this patch adapts TestLooper to allow better control on
delay messages that will be verified in subsequent tests.

Test: atest OffloadControllerTest
Bug: 149467454
Change-Id: Icfd6ff289d6689ae2d5753d3fe472516c808dc7a