Add notice of requesting source code in modulemetadata

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
Change-Id: I87fc25c114f8d80872509a9f4b787c8f3e4ac8ff