[SP27.3] Rename TestableNetworkStatsProvider

Currently, TestableNetworkStatsProvider is a subclass of
INetworkStatsProvider. This is not very accurate naming after
API council review feedback since now we have
NetworkStatsProvider as a system api interface.

This is the counter-part change of actual renaming CL in
NetworkStack.

Test: atest FrameworksNetTests TetheringTests
Bug: 150643374
Change-Id: Ifa8175dc4e2fe2b907ec13b3bd2eca12974f5ea7
Merged-In: Ifa8175dc4e2fe2b907ec13b3bd2eca12974f5ea7
(cherry-picked from ag/10976522)