fix build

getAndroidPrebuild can accept any variable that can be converted
into a string

Change-Id: I199f54458cdef16e55a1283b69643e7b56ded863
diff --git a/build.gradle b/build.gradle
index 65040a5..6a30e07 100644
--- a/build.gradle
+++ b/build.gradle
@@ -168,7 +168,7 @@
  * @param apiLevel the API level or "current"
  * @return a file collection containing the Android prebuilt JAR
  */
-FileCollection getAndroidPrebuilt(String apiLevel) {
+FileCollection getAndroidPrebuilt(apiLevel) {
     files("${project.rootDir}/../../prebuilts/sdk/$apiLevel/android.jar")
 }