RxJava is on the public API so the library needs to declare it as such (#3155)

clients/java/signalr/build.gradle

@@ -14,6 +14,7 @@ plugins {
     id 'maven'
 }
 
+apply plugin: "java-library"
 apply plugin: "com.diffplug.gradle.spotless"
 
 group 'com.microsoft.signalr'
@@ -34,7 +35,7 @@ dependencies {
     testCompile 'org.slf4j:slf4j-jdk14:1.7.25'
     implementation 'com.google.code.gson:gson:2.8.5'
     implementation 'com.squareup.okhttp3:okhttp:3.11.0'
-    implementation 'io.reactivex.rxjava2:rxjava:2.2.2'
+    api 'io.reactivex.rxjava2:rxjava:2.2.2'
     implementation 'org.slf4j:slf4j-api:1.7.25'
 }