[SP27.2] 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.

Thus, this change rename it to TestableNetworkStatsProviderBinder
and create another TestableNetworkStatsProvider to support
subsequent test of NetworkStatsProvider.

Test: atest FrameworksNetTests TetheringTests
Bug: 150643374
Change-Id: I6c359dc718c24c384b827a465980b1c1c24075a0