Lazy-create IpSecTunnelInterface

With the Network(int) constructor moving to mainline, we cannot use the
constructor, and should just create the IpSecTunnelInterface lazily, at
first use.

Bug: 165827287
Test: atest FrameworksVcnTests
Change-Id: Ic01858e0f9c80d316676cfbae5edca114655c3fe