Enable explicit API Mode for download library

and clean up the API a bit in the process

app/src/androidTest/java/org/fdroid/fdroid/net/HttpDownloaderTest.java

@@ -1,10 +1,6 @@
 
 package org.fdroid.fdroid.net;
 
-import static org.junit.Assert.assertFalse;
-import static org.junit.Assert.assertTrue;
-import static org.junit.Assert.fail;
-
 import android.os.Build;
 import android.util.Log;
 
@@ -29,6 +25,10 @@ import java.util.List;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.TimeUnit;
 
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.fail;
+
 public class HttpDownloaderTest {
     private static final String TAG = "HttpDownloaderTest";