android: generate versionCode automatically

It's a number whichb must be ever increasing with each build submitted to the
store.

Automate its value by using the number of seconds since 1st of January 2019.
That should be enough for ~680 years.

android/app/build.gradle

@@ -8,6 +8,11 @@ if (googleServicesEnabled) {
     apply plugin: 'io.fabric'
 }
 
+// Use the number of seconds/10 since Jan 1 2019 as the versionCode.
+// This lets us upload a new build at most every 10 seconds for the
+// next ~680 years.
+// https://stackoverflow.com/a/38643838
+def vcode = (int)(((new Date().getTime()/1000) - 1546297200) / 10)
 
 android {
     compileSdkVersion rootProject.ext.compileSdkVersion
@@ -15,7 +20,7 @@ android {
 
     defaultConfig {
         applicationId 'org.jitsi.meet'
-        versionCode Integer.parseInt(project.buildNumber)
+        versionCode vcode
         versionName project.appVersion
 
         minSdkVersion rootProject.ext.minSdkVersion

android/gradle.properties

@@ -17,6 +17,5 @@
 # http://www.gradle.org/docs/current/userguide/multi_project_builds.html#sec:decoupled_projects
 # org.gradle.parallel=true
 
-buildNumber=1
 appVersion=19.1.0
 sdkVersion=2.0.0