Make generate_gradle.py work with preview SDKs.

build/android/gradle/android.jinja

 {# Copyright 2016 The Chromium Authors. All rights reserved. #}
 {# Use of this source code is governed by a BSD-style license that can be #}
 {# found in the LICENSE file. #}
 {% macro expand_sourceset(variables, prefix) %}
 {% if variables is defined %}
         {{ prefix }} {
             manifest.srcFile "{{ variables.android_manifest }}"
             java.srcDirs = [
 {% for path in variables.java_dirs %}
                 "{{ path }}",
@@ skipping 21 matching lines @@
 {% endmacro %}
 // Generated by //build/android/generate_gradle.py
 
 {% if template_type in ('android_library', 'android_junit') %}
 apply plugin: "com.android.library"
 {% elif template_type == 'android_apk' %}
 apply plugin: "com.android.application"
 {% endif %}
 
 android {
-    compileSdkVersion {{ compile_sdk_version }}
+    compileSdkVersion "{{ compile_sdk_version }}"
     buildToolsVersion "{{ build_tools_version }}"
     publishNonDefault true
     // Multiple targets use the package name "dummy.package"
     enforceUniquePackageName false
 
     compileOptions {
         sourceCompatibility JavaVersion.VERSION_1_7
         targetCompatibility JavaVersion.VERSION_1_7
     }
 
@@ skipping 40 matching lines @@
     }
     tasksToDisable.each { Task task ->
       task.enabled = false
     }
 {% if bootclasspath is defined %}
     tasks.withType(JavaCompile) {
       options.bootClasspath = "{{ bootclasspath }}"
     }
 {% endif %}
 }