Revert "UsbDeviceManager: fix b/21429947 regression"

This causes various problems with our testing infrastructure.

This reverts commit b210026e3d5c955628ca8b8b9191ade08891e9ef.

Bug: 22447614
Bug: 21429947
Change-Id: I57623e3d993e65b6ad89e7a7d28e9575cf638994
diff --git a/services/usb/java/com/android/server/usb/UsbDeviceManager.java b/services/usb/java/com/android/server/usb/UsbDeviceManager.java
index 4cfa37a..1787b91 100644
--- a/services/usb/java/com/android/server/usb/UsbDeviceManager.java
+++ b/services/usb/java/com/android/server/usb/UsbDeviceManager.java
@@ -49,8 +49,10 @@
 import com.android.server.FgThread;
 
 import java.io.File;
+import java.io.FileDescriptor;
 import java.io.FileNotFoundException;
 import java.io.IOException;
+import java.io.PrintWriter;
 import java.util.HashMap;
 import java.util.LinkedList;
 import java.util.List;
@@ -805,7 +807,8 @@
         }
 
         private String getDefaultFunctions() {
-            return mAdbEnabled ? UsbManager.USB_FUNCTION_ADB : UsbManager.USB_FUNCTION_MTP;
+            return SystemProperties.get(USB_PERSISTENT_CONFIG_PROPERTY,
+                    UsbManager.USB_FUNCTION_ADB);
         }
 
         public void dump(IndentingPrintWriter pw) {