Remove use of internal gradle OperatingSystem class.

This CL copies the logic out of OperatingSystem and just
uses it directly to avoid calling into an internal gradle class.

Test: ./gradlew clean still works on ubuntu and my macbook
Change-Id: I47c3af9f8f7d66e050723ee97ff22b799d719b13
diff --git a/buildSrc/init.gradle b/buildSrc/init.gradle
index 7871a0b..91ab17f 100644
--- a/buildSrc/init.gradle
+++ b/buildSrc/init.gradle
@@ -20,7 +20,6 @@
 import com.google.common.base.Charsets
 import com.google.common.io.Files
 
-import org.gradle.internal.os.OperatingSystem
 
 def supportRoot = ext.supportRootFolder
 if (supportRoot == null) {
@@ -56,7 +55,10 @@
 }
 
 def setSdkInLocalPropertiesFile() {
-    final String platform = OperatingSystem.current().isMacOsX() ? 'darwin' : 'linux'
+    final String osName = System.getProperty("os.name").toLowerCase();
+    final boolean isMacOsX =
+            osName.contains("mac os x") || osName.contains("darwin") || osName.contains("osx");
+    final String platform = isMacOsX ? 'darwin' : 'linux'
     System.setProperty('android.dir', "${supportRootFolder}/../../")
     ext.buildToolsVersion = '26.0.0'
     final String fullSdkPath = "${repos.prebuiltsRoot}/fullsdk-${platform}"