[android] #3023 - reactive token generation

[android] #3023 - update documentation to reflect change

android/MapboxGLAndroidSDKTestApp/build.gradle

@@ -15,6 +15,25 @@ apply plugin: 'com.android.application'
 apply plugin: 'io.fabric'
 apply plugin: 'checkstyle'
 
+task accessToken {
+    def tokenFile = new File("MapboxGLAndroidSDKTestApp/src/main/res/values/developer-config.xml")
+    if (!tokenFile.exists()) {
+        String tokenFileContents = "<?xml version=\"1.0\" encoding=\"utf-8\"?>\n" +
+                "<resources>\n" +
+                "    <string name=\"access_token\">" + "$System.env.MAPBOX_ACCESS_TOKEN" + "</string>\n" +
+                "</resources>"
+
+        if (tokenFileContents == null) {
+            throw new InvalidUserDataException("You must set the MAPBOX_ACCESS_TOKEN environment variable.")
+        }
+        tokenFile.write(tokenFileContents)
+    }
+}
+
+gradle.projectsEvaluated {
+    preBuild.dependsOn('accessToken')
+}
+
 android {
     compileSdkVersion 23
     buildToolsVersion "23.0.2"