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