Move to Android gradle plugin 2.3.0

Test: ./gradlew assemble still works
Change-Id: I21a09960e4736b944bafc424f84e8eeb7d70c9ba

dependencies.gradle

@@ -26,7 +26,7 @@ libs.espresso_core = 'com.android.support.test.espresso:espresso-core:2.3-alpha'
 libs.espresso_contrib = 'com.android.support.test.espresso:espresso-contrib:2.3-alpha'
 
 // Keep gradle plugin version in sync with ub_supportlib-master manifest.
-libs.gradle = 'com.android.tools.build:gradle:2.2.4'
+libs.gradle = 'com.android.tools.build:gradle:2.3.0'
 
 // Other dependencies
 libs.xml_parser_apis = 'xerces:xmlParserAPIs:2.6.2'

init.gradle

@@ -76,10 +76,10 @@ def enableDoclavaAndJDiff(p) {
 def setSdkInLocalPropertiesFile() {
     final String platform = OperatingSystem.current().isMacOsX() ? 'darwin' : 'linux'
     System.setProperty('android.dir', "${supportRootFolder}/../../")
+    ext.buildToolsVersion = '26.0.0'
     final String fullSdkPath = "${init.prebuiltsRoot}/fullsdk-${platform}"
     if (file(fullSdkPath).exists()) {
         gradle.ext.currentSdk = 26
-        ext.buildToolsVersion = '26.0.0'
         project.ext.androidJar = files("${fullSdkPath}/platforms/android-${gradle.ext.currentSdk}" +
                 "/android.jar")
         System.setProperty('android.home', "${init.prebuiltsRoot}/fullsdk-${platform}")
@@ -87,7 +87,6 @@ def setSdkInLocalPropertiesFile() {
         props.write "sdk.dir=${fullSdkPath}"
     } else {
         gradle.ext.currentSdk = 'current'
-        ext.buildToolsVersion = '24.0.1'
         project.ext.androidJar = files("${init.prebuiltsRoot}/sdk/current/android.jar")
         File props = file("local.properties")
         props.write "android.dir=../../"