Add a Gradle task rebuilding all API sample jars

The new task is buildApiSampleJars

Change-Id: I69ae58e25aec85b7da57b21993cf7b6b1dc6efec
diff --git a/build.gradle b/build.gradle
index 2d98276..6d767e0 100644
--- a/build.gradle
+++ b/build.gradle
@@ -724,6 +724,12 @@
     destinationDir file('tests')
 }
 
+task buildApiSampleJars {
+    dependsOn buildApiUsageSample
+    dependsOn buildD8ApiUsageSample
+    dependsOn buildR8ApiUsageSample
+}
+
 task buildDebugInfoExamplesDex {
     def examplesDir = file("src/test/java")
     def hostJar = "debuginfo_examples.jar"