[Cherry-pick] Move AttributionNode out of atoms.proto Bug: 253022649 Bug: 386700524 Test: stats-log-api-gen-test Test: m statslog-textclassifier-java-gen (cherry picked from https://android-review.googlesource.com/q/commit:a24ed2ef058d07416bd1c44bf39d195f61992c24) (cherry picked from https://android-review.googlesource.com/q/commit:fdadd87bbc9fc189c393f7e0998eaf10f9a7cb11) Merged-In: Ibe7a3da69110875e3a56f0d9799150f80763c29a Change-Id: Ibe7a3da69110875e3a56f0d9799150f80763c29a