Share GetThresholdValue with global_calibration

Twas hard coded in two spots.  Let's not do that.

Change-Id: I038ef4d6bb840c7558f7652e7073b531695844a7
diff --git a/y2019/vision/target_finder.h b/y2019/vision/target_finder.h
index 4583690..666ae2f 100644
--- a/y2019/vision/target_finder.h
+++ b/y2019/vision/target_finder.h
@@ -25,7 +25,7 @@
   aos::vision::RangeImage Threshold(aos::vision::ImagePtr image);
 
   // Value against which we threshold.
-  uint8_t GetThresholdValue() { return 120; }
+  static uint8_t GetThresholdValue() { return 100; }
 
   // filter out obvious or durranged blobs.
   void PreFilter(BlobList *imgs);