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/image_properties_android.cc b/image_properties_android.cc
index 0e6385d..e815dbf 100644
--- a/image_properties_android.cc
+++ b/image_properties_android.cc
@@ -45,6 +45,7 @@
 // System properties that identifies the "board".
 const char kPropProductName[] = "ro.product.name";
 const char kPropBuildFingerprint[] = "ro.build.fingerprint";
+const char kPropBuildType[] = "ro.build.type";
 
 std::string GetStringWithDefault(const brillo::OsReleaseReader& osrelease,
                                  const std::string& key,
@@ -87,6 +88,9 @@
   property_get(kPropBuildFingerprint, prop, "none");
   result.build_fingerprint = prop;
 
+  property_get(kPropBuildType, prop, "");
+  result.build_type = prop;
+
   // Brillo images don't have a channel assigned. We stored the name of the
   // channel where we got the image from in prefs at the time of the update, so
   // we use that as the current channel if available. During provisioning, there