android compileSdkVersion 26, use buildToolsVersion 26.0.3

Summary:
Starting August 2018, Google Play will require targetSdkVersion 26 for new applications, and November 2018 for application updates.

This PR will use Android build tools 26.0.3 and compilerSdk 26, then support library version 26.0.2 to make targeting 26 easier in the future.

I think this PR will help to people compile and test their applications, thus make transition easier (smoother). Also we'll have opportunity and time to migrate code to target 26.

facebook/react-native#18095

React Native on android must work as usual

Closes facebook/react-native#19257

Differential Revision: D8010354

Pulled By: mdvacca

fbshipit-source-id: 63ba03585e918b38c2a2adb5d2f2e85d7ce46fae

Author: dulmandakh

android/app/build.gradle

@@ -84,8 +84,8 @@ def enableSeparateBuildPerCPUArchitecture = false
 def enableProguardInReleaseBuilds = false
 
 android {
-    compileSdkVersion 23
-    buildToolsVersion "23.0.1"
+    compileSdkVersion 26
+    buildToolsVersion "26.0.3"
 
     defaultConfig {
         applicationId "com.facebook.react.uiapp"
@@ -137,7 +137,7 @@ android {
 
 dependencies {
     compile fileTree(dir: 'libs', include: ['*.jar'])
-    compile 'com.android.support:appcompat-v7:23.0.1'
+    compile 'com.android.support:appcompat-v7:26.0.2'
 
     // Build React Native from source
     compile project(':ReactAndroid')