Sign WebView with platform keys

We are using the prebuilt WebView as default system WebView on builds
without preloaded Google Chrome. Therefore, it needs be signed with
release keys in release builds.

Issue: FP2P-461
Change-Id: I6c56a9b9cf9c811a0ceb6772bd469ff1352b8de2
diff --git a/Android.mk b/Android.mk
index f4c3fea..8207efc 100644
--- a/Android.mk
+++ b/Android.mk
@@ -23,7 +23,7 @@
 
 LOCAL_MODULE := webview
 LOCAL_MODULE_CLASS := APPS
-LOCAL_CERTIFICATE := $(DEFAULT_SYSTEM_DEV_CERTIFICATE)
+LOCAL_CERTIFICATE := platform
 LOCAL_REQUIRED_MODULES := \
         libwebviewchromium_loader \
         libwebviewchromium_plat_support