[roll] Roll fuchsia [build][idk] Move IDK content definitions to idk_contents.gni

Create scopes containing the atoms that make up each IDK to
idk_contents.gni where they can be reused. The content definitions are
unchanged.

This is the first step toward allowing the scopes to be reused by
`idk_rules()` instances whose contents are minor variations of the
Fuchsia IDK.

Original-Bug: 393154095
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1267228
Original-Revision: d4df6f22027cfe9797004df50f3c1ac50f54a738
GitOrigin-RevId: c1d902db9dc36e168bdbb1853da44c788c10d7bc
Change-Id: I34e954af6e8632f809b9ca6a1ef34b2261d6d71c

stem

diff --git a/stem b/stem
index cd206b2..752a889 100644
--- a/stem
+++ b/stem
@@ -8,7 +8,7 @@
              root=""
              remote="https://fuchsia.googlesource.com/fuchsia"
              gerrithost="https://fuchsia-review.googlesource.com"
-             revision="5f38b926e60f6b958637a4ffaca5ddcd2d5be21c"
+             revision="d4df6f22027cfe9797004df50f3c1ac50f54a738"
              />
   </imports>
   <projects>