update build tools to 28.0.3
This fixes another warning when updating the android-gradle-plugin to 3.4.2. This is the error mesage that you get otherwise.
The specified Android SDK Build Tools version (27.0.3) is ignored, as it is below the minimum supported version (28.0.3) for Android Gradle Plugin 3.4.2. Android SDK Build Tools 28.0.3 will be used. To suppress this warning, remove "buildToolsVersion '27.0.3'" from your build.gradle file, as each version of the Android Gradle Plugin now has a default version of the build tools.