Add test keys for networkstack

The networkstack is using shared test keys for NetworkStack.apk,
CaptivePortalLogin.apk and NetworkStackPermissionStub.apk.

This allows NetworkStackPermissionStub.apk to define signature
permissions without the platform certificate, and enforce that the other
packages match the signature to use the permissions if they are
installed.

Bug: 124033493
Test: m
Change-Id: I7257a472d702e82acdadffc4e0535c63a8bd591d