android: update app / sdk version

Set them to the next release versions. In additon, the buildNumber variable will
be used to match the requirements of versionCode:
https://developer.android.com/studio/publish/versioning

that is, a monotonically increasing number, independent of the app / sdk
version.

android/app/build.gradle

 
     defaultConfig {
         applicationId 'org.jitsi.meet'
-        versionCode Integer.parseInt("${version}")
-        versionName "1.9.${version}"
+        versionCode Integer.parseInt(project.buildNumber)
+        versionName project.appVersion
 
         minSdkVersion rootProject.ext.minSdkVersion
         targetSdkVersion rootProject.ext.targetSdkVersion

android/gradle.properties

 # http://www.gradle.org/docs/current/userguide/multi_project_builds.html#sec:decoupled_projects
 # org.gradle.parallel=true
 
-version=1
+buildNumber=2
+appVersion=1.21.0
+sdkVersion=1.21.0

android/sdk/build.gradle

         aarArchive(MavenPublication) {
             groupId 'org.jitsi.react'
             artifactId 'jitsi-meet-sdk'
-            version '1.9.0'
+            version project.sdkVersion
 
             artifact("${project.buildDir}/outputs/aar/${project.name}-release.aar") {
                 extension "aar"