Track default upstream when system is ready

Start tracking default upstream from boot.This is useful for
entitlement refine in following change. EntitlementManager can
decide if it needs to process entitlement provisioning before
tethering started.

Test: -atest FrameworksNetTests
      -build, flash, booted
      -manually turnoff/on tethering with different upstream
bug: 111490073

Change-Id: I8fdbd64c52f26b5363693bb5bd8050930e8ea961