Bump compileSdkVersion to 27 and buildToolsVersion to 27.0.3 (#20777)

Summary:
I found that android support library 27.x (facebook/react-native@874cca1) requires compileSdkVersion to be 27. Also found that many FB projects use SDK 27.
Pull Request resolved: facebook/react-native#20777

Differential Revision: D9478431

Pulled By: hramos

fbshipit-source-id: ca100f6b5b39e7d112926124423f9510a0efc291

android/app/build.gradle

@@ -84,8 +84,8 @@ def enableSeparateBuildPerCPUArchitecture = false
 def enableProguardInReleaseBuilds = true
 
 android {
-    compileSdkVersion 26
-    buildToolsVersion "26.0.3"
+    compileSdkVersion 27
+    buildToolsVersion "27.0.3"
 
     defaultConfig {
         applicationId "com.facebook.react.uiapp"