Force all Kotlin artifacts on the classpath to be the expected version.

Test: gw --stop && gw clean (Observe absence of Kotlin version warning)
Change-Id: If03bae18de2e86f1adc3df532374a162d1180d7c
diff --git a/buildSrc/build_dependencies.gradle b/buildSrc/build_dependencies.gradle
index 313c4e3..f693884 100644
--- a/buildSrc/build_dependencies.gradle
+++ b/buildSrc/build_dependencies.gradle
@@ -14,6 +14,13 @@
  * limitations under the License.
  */
 
+def build_versions = [:]
+
+build_versions.kotlin = '1.2.0'
+
+rootProject.ext['build_versions'] = build_versions
+
+
 def build_libs = [:]
 
 def androidPluginVersionOverride = System.getenv("GRADLE_PLUGIN_VERSION")
@@ -31,7 +38,9 @@
 build_libs.jacoco = 'org.jacoco:org.jacoco.core:0.7.8'
 build_libs.jacoco_ant = 'org.jacoco:org.jacoco.ant:0.7.8'
 build_libs.jetifier = 'androidx.tools.jetifier:gradle-plugin:0.1'
-build_libs.kotlin = [gradle_plugin: "org.jetbrains.kotlin:kotlin-gradle-plugin:1.2.0"]
+build_libs.kotlin = [
+        gradle_plugin: "org.jetbrains.kotlin:kotlin-gradle-plugin:${build_versions.kotlin}"
+]
 // jdiff dependencies
 build_libs.jdiff = 'com.android:jdiff:1.1.0'
 build_libs.xml_parser_apis = 'xerces:xmlParserAPIs:2.6.2'