Make requestRouteToHost a no-op for system callers.

Everything in the system should now be using proper multinetwork
APIs instead of this insecure and error-prone API.

Make this method do nothing when called by the system. For now,
keep the code around for backwards compatibility for apps
targeting Android releases before M.

Bug: 25824776
Bug: 25876485
Test: FrameworksNetTests pass
Test: CtsNetTestCasesLegacyApi22 pass
Test: CtsNetTestCasesLegacyPermission22 pass
Test: android.net.cts.ConnectivityManagerTest passes
Change-Id: I9b3557faccccc95c7b954db6a13b853b4c7edea0