[Android] Fix Android Studio 3.0 Beta 7

android/app/build.gradle

 
 android {
     compileSdkVersion rootProject.ext.compileSdkVersion
-    buildToolsVersion rootProject.ext.buildToolsVersion
 
     defaultConfig {
         applicationId 'org.jitsi.meet'

android/build.gradle

         maven { url 'https://maven.google.com' }
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:3.0.0-beta6'
+        classpath 'com.android.tools.build:gradle:3.0.0-beta7'
 
         // NOTE: Do not place your application dependencies here; they belong
         // in the individual module build.gradle files.
 
             project.android {
                 compileSdkVersion rootProject.ext.compileSdkVersion
-                buildToolsVersion rootProject.ext.buildToolsVersion
+                if (rootProject.ext.has('buildToolsVersion')) {
+                    buildToolsVersion rootProject.ext.buildToolsVersion
+                }
                 defaultConfig {
                     minSdkVersion rootProject.ext.minSdkVersion
                     targetSdkVersion rootProject.ext.targetSdkVersion
 
 ext {
     compileSdkVersion = 25
-    buildToolsVersion = "25.0.3"
     minSdkVersion    = 16
     targetSdkVersion = 25
 
 // modules that we utilize such as react-native-background-timer.
 subprojects { subproject ->
     afterEvaluate{
-        if (subproject.plugins.hasPlugin('android')
-                || subproject.plugins.hasPlugin('android-library')) {
+        if ((subproject.plugins.hasPlugin('android')
+                    || subproject.plugins.hasPlugin('android-library'))
+                && rootProject.ext.has('buildToolsVersion')) {
             android {
                 buildToolsVersion rootProject.ext.buildToolsVersion
             }

android/sdk/build.gradle

 
 android {
     compileSdkVersion rootProject.ext.compileSdkVersion
-    buildToolsVersion rootProject.ext.buildToolsVersion
 
     defaultConfig {
         minSdkVersion rootProject.ext.minSdkVersion