Android: Use the git rev-list --count for versionCode

Source/Android/app/build.gradle

@@ -25,8 +25,7 @@ android {
         minSdkVersion 21
         targetSdkVersion 25
 
-        // TODO This should be set to the Buildbot build number for release builds, and be "1" for debug builds.
-        versionCode 13
+        versionCode(getBuildVersionCode())
 
         versionName "${getVersion()}"
     }
@@ -113,3 +112,16 @@ def getVersion() {
 
     return versionNumber
 }
+
+
+def getBuildVersionCode() {
+    try {
+        def versionNumber = 'git rev-list --first-parent --count HEAD'.execute([], project.rootDir).text
+                .trim()
+        return Integer.valueOf(versionNumber);
+    } catch (Exception e) {
+        logger.error('Cannot find git, defaulting to dummy version number')
+    }
+
+    return 0;
+}