[NS A12] Move some legacy type tracker handling to a function

It's fine to do this out of the if() clause because :
• If the network newly satisfies a request it is sure to have
  it in the list of requests it satisfies
• If it does not newly satisfy a request and there is still
  a request with a legacy type that it satisfies, then it
  is already remembered by LegacyTypeTracker

As for the VPN, the code always enters the condition anyway.

Test: ConnectivityServiceTest
Change-Id: I8bd668ad27043d6a5036b1b3c52fa5a3146abcfa