Add Guava dep to exo-workmanager module

This fixes the "cannot access ListenableFuture" build error, even though
it seems on the surface like we shouldn't need a Guava dependency here.

There's more info about what's going on here:
https://blog.gradle.org/guava

PiperOrigin-RevId: 312712991

extensions/workmanager/build.gradle

@@ -35,6 +35,10 @@ android {
 dependencies {
     implementation project(modulePrefix + 'library-core')
     implementation 'androidx.work:work-runtime:2.3.4'
+    // Guava & Gradle interact badly, and this prevents
+    // "cannot access ListenableFuture" errors [internal b/157225611].
+    // More info: https://blog.gradle.org/guava
+    implementation 'com.google.guava:guava:' + guavaVersion
     compileOnly 'org.jetbrains.kotlin:kotlin-annotations-jvm:' + kotlinAnnotationsVersion
 }