Prevent going through unnecessary networks

The outer interface is taken in the constructor of
LegacyVpnRunner. The interface is used to set the type of
connection type. One interface should not map to more than one
network. Go through all the network is unnecessary if the target
connection is found. Thus, skip other networks if target network
is found.

Bug: 169215213
Test: atest FrameworksNetTests
Test: build and manually test to add legacy vpn
Change-Id: I31096d07177f5ae168becb1b4311f7b8eeac855a