Report build_type to Omaha.

Send additional os_build_type <app> attribute to distinguish between
user and userdebug images.

Bug: 36491462
Test: check omaha request xml
Change-Id: I519e85293db9a2194e0cd015a3a944adc723c3dd
(cherry picked from commit eba2297072914582aed5008ad5f99374124ae4d7)
diff --git a/omaha_request_params.h b/omaha_request_params.h
index cdc5f38..f8e9438 100644
--- a/omaha_request_params.h
+++ b/omaha_request_params.h
@@ -105,6 +105,7 @@
   inline std::string os_build_fingerprint() const {
     return image_props_.build_fingerprint;
   }
+  inline std::string os_build_type() const { return image_props_.build_type; }
   inline std::string board_app_id() const { return image_props_.product_id; }
   inline std::string canary_app_id() const {
     return image_props_.canary_product_id;