Expose NETWORK_STACK permission for system and test API

NETWORK_STACK permission is used for system and test API
permission check which should also be system API. Expose
as it is.

Bug: 148379628
Test: make test-api-stubs-docs-update-current-api \
      system-api-stubs-docs-update-current-api

Change-Id: I87db8568f20f26eabf236f76a8e281a63bf25114
Merged-In: I27061280b0c49677fa651c6f9799e48b78e4c109