Add r8lib target for performance testing

Change-Id: If40952ee64d008ecfff45371db299f135e1e6b8c
diff --git a/build.gradle b/build.gradle
index 93d7e1b..7ff46eb 100644
--- a/build.gradle
+++ b/build.gradle
@@ -702,6 +702,11 @@
     workingDir = projectDir
 }
 
+task R8LibApiOnly {
+    dependsOn r8LibCreateTask("Api", "src/main/keep.txt", R8NoManifest, r8LibPath)
+    outputs.file r8LibPath
+}
+
 task R8LibNoDeps {
     dependsOn r8LibCreateTask(
             "NoDeps",