Update project.properties to use proper SDK 21 instead of preview L.

Change-Id: I737bdfcfea80a6e722cebf50a2f1ad67542690e1
diff --git a/project.properties b/project.properties
index 2f095e1..6e18427 100644
--- a/project.properties
+++ b/project.properties
@@ -11,4 +11,4 @@
 #proguard.config=${sdk.dir}/tools/proguard/proguard-android.txt:proguard-project.txt
 
 # Project target.
-target=android-L
+target=android-21