Change the gradle version code to use better logic

The previous version would check any tag in the repo and not just from
the ones that are from the current commit. This would ruin, in the
future, doing reproducible builds.

app/build.gradle

@@ -6,7 +6,7 @@ def getVersionCode = { ->
     try {
         def code = new ByteArrayOutputStream()
         exec {
-            commandLine 'git', 'tag', '--list', 'v*'
+            commandLine 'git' 'rev-list' 'HEAD' '--first-parent' '--count'
             standardOutput = code
         }
         return code.toString().split("\n").size()