move minSdkVersion and targetSdkVersion to build.gradle
Moving minSdkVersion and targetSdkVersion from the manifest to build.gradle is recommended here: https://developer.android.com/studio/publish/versioning.html#minsdkversion
This will also make life easier when upgrading the android-gradle-plugin. Higher versions of the android-gradle-plugin throw an error that require removing the minSdkVersion from the manifest.