Add an API dependency onto guava listenablefuture.

Since AppSearch methods return this class, without the API
dependency, developers see class resolution errors unless they
add the following dep to their own app:
  'com.google.guava:listenablefuture:1.0'.
or, guava itself:
  'com.google.guava:guava:30.1.1-android'

This CL avoids needing to do this.

Test: Manually created an app with a project dependency.
Change-Id: I85de80a77e2dc4ba3aaa6eac37a024df65cfaa40
diff --git a/appsearch/appsearch/build.gradle b/appsearch/appsearch/build.gradle
index 49708af..3f29de2 100644
--- a/appsearch/appsearch/build.gradle
+++ b/appsearch/appsearch/build.gradle
@@ -31,6 +31,7 @@
 
 dependencies {
     api('androidx.annotation:annotation:1.1.0')
+    api(libs.guavaListenableFuture)
 
     implementation('androidx.concurrent:concurrent-futures:1.0.0')
     implementation('androidx.core:core:1.2.0')