Add notice of requesting source code in networkstack

Flag: EXEMPT license change for b/418865751
Bug: 418865751
Test: presubmits
Test: lunch husky-trunk_staging-eng && m, an extra line is added at the top of NOTICE.html file in NOTICE.html.gz
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:2696c3acde268c6b491753e9636ccb2a7d30c6c0)
Merged-In: I452dbb664504c75805c83a0014db9d25ea6cf7b7
Change-Id: I452dbb664504c75805c83a0014db9d25ea6cf7b7