title: Gradle doesn't build external project
body: <p>Gradle doesn't seem to be building a project I am including:</p>
  
  <pre><code>FATAL EXCEPTION: main
       Process: com.projecttango.examples.cpp.planefitting, PID: 14678 
       java.lang.UnsatisfiedLinkError: dalvik.system.PathClassLoader[DexPathList[[zip file "/system/framework/libtango_device2.jar", zip file "/data/app/com.projecttango.examples.cpp.planefitting-1/base.apk"],nativeLibraryDirectories=[/data/app/com.projecttango.examples.cpp.planefitting-1/lib/arm, /data/app/com.projecttango.examples.cpp.planefitting-1/base.apk!/lib/armeabi-v7a, /vendor/lib, /system/lib]]] couldn't find "libcpp_plane_fitting_example.so"
  </code></pre>
  
  <p>In my main <code>build.gradle</code>:</p>
  
  <pre><code>dependencies {
      compile project(':cpp_example_util')
  }
  </code></pre>
  
  <p>My top-level <code>build.gradle</code>:</p>
  
  <pre><code>buildscript {
  
      repositories {
          jcenter()
      }
  
      dependencies {
          classpath 'com.android.tools.build:gradle:2.2.2'
      }
  }
  
  allprojects {
      repositories {
          jcenter()
      }
  }
  task clean(type: Delete) {
      delete rootProject.buildDir
  }
  </code></pre>
  
  <p>The <code>build.gradle</code> of my external project:</p>
  
  <pre><code>apply plugin: 'com.android.library'
  
  android {
      compileSdkVersion 19
      buildToolsVersion "24"
  
      defaultConfig {
          minSdkVersion 17
          targetSdkVersion 19
      }
  
      buildTypes {
          release {
              minifyEnabled false
              proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.txt'
          }
      }
  }
  
  def external_lib_prefix = null
  if (project.hasProperty("Tango.catkin_devel_prefix")) {
      external_lib_prefix = project.property("Tango.catkin_devel_prefix")
  }
  </code></pre>
  
  <p>The <code>settings.gradle</code>:</p>
  
  <pre><code>include ':app'
  include ':cpp_example_util'
  project(':cpp_example_util').projectDir = new File('../cpp_example_util/app')
  </code></pre>
  
  <p>As shown in the error, there is no <code>libcpp_plane_fitting_example</code> library being built. </p>