Add RxJava2 helper module.

Add RxJava2 helper module to support RxJava2 APIs
Add RxJava2 adapter.

Relnote: Add an adapter to support RxJava2
Bug: 159400986
Test: ./gradlew window:window-rxjava2:cAT
Change-Id: Ib7bd39e247ee4b96e9db13e723736241b58fec5f
diff --git a/docs-tip-of-tree/build.gradle b/docs-tip-of-tree/build.gradle
index 046930b..ddf2b02 100644
--- a/docs-tip-of-tree/build.gradle
+++ b/docs-tip-of-tree/build.gradle
@@ -273,6 +273,7 @@
     docs(project(":webkit:webkit"))
     docs(project(":window:window"))
     docs(project(":window:window-java"))
+    docs(project(":window:window-rxjava2"))
     stubs(project(":window:window-sidecar"))
     stubs(project(":window:window-extensions"))
     docs(project(":work:work-gcm"))