Convert three variable assignments to the usage of compound operators

[elfring]

:eyes: Some source code analysis tools can help to find opportunities for improving software components. :thought_balloon: I propose to increase the usage of compound operators accordingly.

diff --git a/app/src/main/org/runnerup/util/Formatter.java b/app/src/main/org/runnerup/util/Formatter.java
index bc4f5a60..ac4d3e02 100644
--- a/app/src/main/org/runnerup/util/Formatter.java
+++ b/app/src/main/org/runnerup/util/Formatter.java
@@ -565,7 +565,7 @@ public class Formatter implements OnSharedPreferenceChangeListener {
             str = DateUtils.formatElapsedTime(val);
         }
         if (includeUnit) {
-            str = str + " /" + resources.getString((metric ? R.string.metrics_distance_km : R.string.metrics_distance_mi));
+            str += " /" + resources.getString((metric ? R.string.metrics_distance_km : R.string.metrics_distance_mi));
         }
         return str;
     }
diff --git a/app/src/main/org/runnerup/util/GraphWrapper.java b/app/src/main/org/runnerup/util/GraphWrapper.java
index 51e3dd8f..cee8b492 100644
--- a/app/src/main/org/runnerup/util/GraphWrapper.java
+++ b/app/src/main/org/runnerup/util/GraphWrapper.java
@@ -449,7 +449,7 @@ public class GraphWrapper implements Constants {
                         sum += aHrzHist;
                     }
                     for (int i = 0; i < hrzHist.length; i++) {
-                        hrzHist[i] = hrzHist[i] / sum;
+                        hrzHist[i] /= sum;
                         System.err.print(" " + hrzHist[i]);
                     }
                     Log.e(getClass().getName(), "\n");
@@ -489,7 +489,7 @@ public class GraphWrapper implements Constants {
 
         for (int aData : data) {
             if (aData > 0) {
-                sum = sum + aData;
+                sum += aData;
                 no++;
             }
         }