Add NetworkStackPermissionStub definitions

The NetworkStackPermissionStub package is used to enforce that
permissions used by the NetworkStack are only used in packages
sharing signature with NetworkStackPermissionStub.

Permissions defined in this package are intended to be used only by the
NetworkStack: both NetworkStack and the stub APK will be signed with
a dedicated certificate to ensure that, with permissions being signature
permissions.

This APK *must* be installed, even if the NetworkStack app is not
installed, because otherwise, any application will be able to define
this permission and the system will give that application full access
to the network stack.

Test: flashed, booted
Bug: 112869080
Change-Id: Ia13a9e6a703cb7b4403697a7f7bfff0f6f3b813e