Set proguard flags

Without it, email won't work properly and many of tests will fail.

Bug 2482363
diff --git a/Android.mk b/Android.mk
index 794feec..e7c3d42 100644
--- a/Android.mk
+++ b/Android.mk
@@ -28,6 +28,8 @@
 
 LOCAL_PACKAGE_NAME := Email
 
+LOCAL_PROGUARD_FLAGS := -include $(LOCAL_PATH)/proguard.flags
+
 include $(BUILD_PACKAGE)
 
 # additionally, build unit tests in a separate .apk