Disable building the APK when a prebuilt is in use.

If the product is configured to use a prebuilt webview APK, don't
attempt to build it from source to avoid a duplicate definition.

Bug: 16870861
Change-Id: I689cda55ef75c843bca2aa020df1b4cefd8821dc
(cherry picked from commit c182da21c498c2b3f143e9b775f89a7aeb9be6ce)
diff --git a/chromium/Android.mk b/chromium/Android.mk
index a624f50..edcfdb2 100644
--- a/chromium/Android.mk
+++ b/chromium/Android.mk
@@ -74,9 +74,11 @@
 include $(BUILD_SHARED_LIBRARY)
 
 
-# Include the makefile for the main package.
+# Include the makefile for the main package unless we are using a prebuilt.
+ifneq ($(PRODUCT_PREBUILT_WEBVIEWCHROMIUM),yes)
 extra_java_files :=
 include $(LOCAL_PATH)/package.mk
+endif
 
 # Build other stuff
 include $(call first-makefiles-under,$(LOCAL_PATH))