Do not install libsampleplugin by default.

This avoids installing the library during a normal build. When building
SampleBrowserPlugin, the library will be packaged in the apk.
diff --git a/samples/BrowserPlugin/jni/Android.mk b/samples/BrowserPlugin/jni/Android.mk
index b888d54..cbd8a15 100644
--- a/samples/BrowserPlugin/jni/Android.mk
+++ b/samples/BrowserPlugin/jni/Android.mk
@@ -53,12 +53,12 @@
 LOCAL_SHARED_LIBRARIES := \
 	libnativehelper
 
-LOCAL_SRC_FILES := $(LOCAL_SRC_FILES)
 LOCAL_CFLAGS += -fvisibility=hidden 
 LOCAL_PRELINK_MODULE:=false
-LOCAL_MODULE_CLASS := SHARED_LIBRARIES
 
 LOCAL_MODULE:= libsampleplugin
 
+LOCAL_MODULE_TAGS := optional
+
 include $(BUILD_SHARED_LIBRARY)