Add build targets for non-mainline NetworkStack

Add PlatformCaptivePortalLogin and PlatformNetworkPermissionConfig build
targets to override CaptivePortalLogin and NetworkPermissionConfig with
platform-key-signed packages easily. The added packages are unused by
default but can simplify setup for OEMs that use InProcessNetworkStack.

Bug: 132635211
Test: Added the new packages to sailfish build, built image: device has
      the proper packages signed with the right keys, networking and
      captive portal working fine.

Change-Id: I09656deb6ba928f56b478adebb60268fa6f484b0