Add notice of requesting source code in docsui

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:c5648001fceca0f0027c92c09238490c4840c6f2)
Merged-In: I1b5cf58f4f69c2dbe80f3881e518a56560a20564
Change-Id: I1b5cf58f4f69c2dbe80f3881e518a56560a20564