Split-tunnel VPN (which are the only ones affected by this change)
always fall through to the default network for routes they don't
handle, and even if the underlying network(s) don't provide access
this may be a pinhole that can actually reach the broader network.
In practice this behaves like the original release of P and is the
safest thing to do for Q. In R we should evaluate giving the VPN
app the ability to simply tell the network stack whether it does
provide Internet access or not.
Bug: 119216095
Test: FrameworksNetTests NetworkStackTests
Change-Id: I262ca41fe0225660551c9a421562405366b6acac
318feaf1ed