Upgrade example build (dropbox#339)

* example: Guard against ndkDir being null at configuration time

This is a follow-up to PR dropbox#329.

* example: Upgrade to Android Gradle plugin 3.0.1

Each version of the Android Gradle Plugin now has a default version of the
build tools. The "compile" configuration was renamed to "implementation".

* example: Upgrade to Gradle 4.4.1

example/android/app/build.gradle

@@ -2,7 +2,6 @@ apply plugin: 'com.android.application'
 
 android {
     compileSdkVersion 23
-    buildToolsVersion "26.0.0"
 
     defaultConfig {
         applicationId "com.dropbox.textsort"
@@ -27,33 +26,37 @@ android {
 }
 
 dependencies {
-    compile fileTree(dir: 'libs', include: ['*.jar'])
-    compile 'com.google.code.findbugs:jsr305:3.0.0'
+    implementation fileTree(dir: 'libs', include: ['*.jar'])
+    implementation 'com.google.code.findbugs:jsr305:3.0.0'
 }
 
 task ndkBuild(type: Exec) {
-    File ndkDir = project.android.ndkDirectory
-    def ndkBuildExecutable = new File(ndkDir, 'ndk-build')
+    def ndkDir = project.android.ndkDirectory
+    def ndkBuildExecutable
+
+    if (ndkDir) {
+        ndkBuildExecutable = new File(ndkDir, 'ndk-build')
+        def MainDirectory = System.getProperty("user.dir") + '/app/'
+
+        environment("NDK_PROJECT_PATH", MainDirectory)
+        environment("GYP_CONFIGURATION", "Release")
+
+        commandLine ndkBuildExecutable
+    }
 
     doFirst {
-        if (ndkDir == null) {
+        if (!ndkDir) {
             def gradle_project_root = project.rootProject.rootDir
             throw new GradleException("NDK is not configured. Make sure there is a local.properties " +
                     "file with an ndk.dir entry in the directory ${gradle_project_root}.")
         }
 
-        if (!ndkBuildExecutable.exists()) {
+        if (!ndkBuildExecutable.file) {
             throw new GradleException("Could not find ndk-build. The configured NDK directory ${ndkDir} may not be correct.")
         }
     }
-
-    String MainDirectory = System.getProperty("user.dir") + '/app/'
-    environment("NDK_PROJECT_PATH", MainDirectory)
-    environment("GYP_CONFIGURATION", "Release")
-    commandLine ndkBuildExecutable
 }
 
 tasks.withType(JavaCompile) {
     compileTask -> compileTask.dependsOn ndkBuild
 }
-

example/android/build.gradle

@@ -2,10 +2,11 @@
 
 buildscript {
     repositories {
+        google()
         jcenter()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:2.3.3'
+        classpath 'com.android.tools.build:gradle:3.0.1'
 
         // NOTE: Do not place your application dependencies here; they belong
         // in the individual module build.gradle files

example/android/gradle/wrapper/gradle-wrapper.properties

@@ -2,4 +2,4 @@ distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-4.2-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-4.4.1-all.zip