use defined gradle_tools variable and update to 3.1.3 (android10#36)

Author: onyxmueller

build.gradle

@@ -3,7 +3,7 @@ apply from: 'buildsystem/dependencies.gradle'
 
 buildscript {
  ext.kotlin_version = '1.2.31'
- ext.gradle_tools = '3.1.0'
+ ext.gradle_tools = '3.1.3'
  ext.build_tools = '27.0.3'
 
  ext.compile_sdk = 27
@@ -21,7 +21,7 @@ buildscript {
  }
 
  dependencies {
- classpath 'com.android.tools.build:gradle:3.1.3'
+ classpath "com.android.tools.build:gradle:$gradle_tools"
  //noinspection DifferentKotlinGradleVersion
  classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
  }