[NS A14] Move code notifying battery stats in its right place

This should be done once every time an interface comes online.
Doing this in updateLinkProperties guarantees this happens every
time a new interface comes online, but it doesn't do it more
often than needed.

Test: FrameworksNetTests NetworkStackTests
Change-Id: I0613c23f44192944266d76107308da8d1c541d1c