Change the const for the unreachable network

Changing the const used for the unreachable network for per-app
functionality for those apps whose traffic for the default network we
wish to lose connectivity as dictated by per app network preferences.
This is being done as per changes in https://r.android.com/1643182.

Bug: 181579204
Test: atest FrameworksNetTests
Change-Id: I1bffadfa03a6221096100c9e0457ea00f94027a3