gradle文件夹 - gradle-wrapper.properties 中指的是gradle 版本号
build.gradle文件 - classpath ‘com.android.tools.build:gradle:3.1.2’ 指的是 gradle 插件的版本号，而不是 gradle 的版本号。
不过 gradle 插件的版本号和 gradle 的版本号又是有关联的，关系如下：
续：下面是详细gradle插件版本与gradle版本更新日志（至2019.1.10）：
来自：https://developer.android.google.cn/studio/releases/gradle-plugin#updating-plugin
2.2.0 (September 2016)
Gradle 2.14.1 or higher.
Build Tools 23.0.2 or higher.
2.3.0 (February 2017)
Gradle 3.3 or higher.
Build Tools 25.0.0 or higher.
3.0.0 (October 2017)
Gradle 4.1 or higher. To learn more, read the section about updating Gradle.
Build Tools 26.0.2 or higher. With this update, you no longer need to specify a version for the build tools—the plugin uses the minimum required version by default. So, you can now remove the android.buildToolsVersion property.
3.1.0 (March 2018)
Gradle 4.4 or higher. To learn more, read the section about updating Gradle.
Build Tools 27.0.3 or higher. Keep in mind, you no longer need to specify a version for the build tools using the android.buildToolsVersion property—the plugin uses the minimum required version by default.
3.2.0 (September 2018)
This version of the Android plugin requires the following:
Gradle 4.6 or higher. To learn more, read the section about updating Gradle.
SDK Build Tools 28.0.3 or higher.
3.2.1 (October 2018)
With this update, you no longer need to specify a version for the SDK Build Tools. The Android Gradle plugin now uses version 28.0.3 by default.
