update build tools versions

Summary:
Update build tools version to 28.0.3 to fix the below warning

WARNING: The specified Android SDK Build Tools version (26.0.2) is ignored, as it is below the minimum supported version (27.0.3) for Android Gradle Plugin 3.1.0.
Android SDK Build Tools 27.0.3 will be used.

Reviewed By: passy

Differential Revision: D17343575

fbshipit-source-id: aa12bf2b55666aacb18f09b1cca22eab05f38220

build.gradle

@@ -34,7 +34,7 @@ ext {
     minSdkVersion = 14
     targetSdkVersion = 25
     compileSdkVersion = 26
-    buildToolsVersion = '26.0.2'
+    buildToolsVersion = '28.0.3'
     sourceCompatibilityVersion = JavaVersion.VERSION_1_7
     targetCompatibilityVersion = JavaVersion.VERSION_1_7
 }