gradle to set project-wide properties (#19845)

Summary:
Change HelloWorld app template to use project-wide properties. See https://developer.android.com/studio/build/gradle-tips#configure-project-wide-properties.

RN community and third-party native module developers adopted this, so it'll make RN package development and usage easy.

CI is green https://circleci.com/gh/dulmandakh/react-native/507
Closes https://github.com/facebook/react-native/pull/19845

Differential Revision: D8675341

Pulled By: hramos

fbshipit-source-id: 4ed06cd7ef7ae70f153d7f335ef8a9d1371f6ce0

local-cli/templates/HelloWorld/android/app/build.gradle

@@ -94,13 +94,13 @@ def enableSeparateBuildPerCPUArchitecture = false
 def enableProguardInReleaseBuilds = false
 
 android {
-    compileSdkVersion 26
-    buildToolsVersion "26.0.3"
+    compileSdkVersion rootProject.ext.compileSdkVersion
+    buildToolsVersion rootProject.ext.buildToolsVersion
 
     defaultConfig {
         applicationId "com.helloworld"
-        minSdkVersion 16
-        targetSdkVersion 22
+        minSdkVersion rootProject.ext.minSdkVersion
+        targetSdkVersion rootProject.ext.targetSdkVersion
         versionCode 1
         versionName "1.0"
         ndk {
@@ -138,7 +138,7 @@ android {
 
 dependencies {
     compile fileTree(dir: "libs", include: ["*.jar"])
-    compile "com.android.support:appcompat-v7:26.1.0"
+    compile "com.android.support:appcompat-v7:${rootProject.ext.supportLibVersion}"
     compile "com.facebook.react:react-native:+"  // From node_modules
 }
 

local-cli/templates/HelloWorld/android/build.gradle

@@ -30,3 +30,11 @@ allprojects {
         }
     }
 }
+
+ext {
+    buildToolsVersion = "26.0.3"
+    minSdkVersion = 16
+    compileSdkVersion = 26
+    targetSdkVersion = 26
+    supportLibVersion = "26.1.0"
+}