metricsd: Rename build_target_id to product_id.

We use product_id instead of build_target_id, the protobuffer should
rename that field too to be consistent.
The same change was made on the backend.

BUG: 24712895
Change-Id: Id9492e3debab986c9df7d4ec0caef8730eb14240
diff --git a/uploader/proto/system_profile.proto b/uploader/proto/system_profile.proto
index 4cab0d9..bac828b 100644
--- a/uploader/proto/system_profile.proto
+++ b/uploader/proto/system_profile.proto
@@ -88,7 +88,7 @@
   optional string application_locale = 4;
 
   message BrilloDeviceData {
-    optional string build_target_id = 1;
+    optional string product_id = 1;
   }
   optional BrilloDeviceData brillo = 21;
 
diff --git a/uploader/system_profile_cache.cc b/uploader/system_profile_cache.cc
index 2437b56..1d87be5 100644
--- a/uploader/system_profile_cache.cc
+++ b/uploader/system_profile_cache.cc
@@ -136,7 +136,7 @@
   profile_proto->set_channel(profile_.channel);
   metrics::SystemProfileProto_BrilloDeviceData* device_data =
       profile_proto->mutable_brillo();
-  device_data->set_build_target_id(profile_.product_id);
+  device_data->set_product_id(profile_.product_id);
 
   return true;
 }