Add api21 source to gradle build for prefs-leanback

b/22404789

Change-Id: I337910b51e63fc41d46dc962dc0c89fdbc4d1a33
diff --git a/v17/preference-leanback/build.gradle b/v17/preference-leanback/build.gradle
index e8195da..4063122 100644
--- a/v17/preference-leanback/build.gradle
+++ b/v17/preference-leanback/build.gradle
@@ -34,7 +34,7 @@
 
     sourceSets {
         main.manifest.srcFile 'AndroidManifest.xml'
-        main.java.srcDir 'src'
+        main.java.srcDirs = ['src', 'api21']
         main.res.srcDir 'res'
         main.assets.srcDir 'assets'
         main.resources.srcDir 'src'