Enabling Android instrumentaiton tests on Travis CI. This was not working at the time due to Travis not being able to boot up emulator or similar.

.travis.yml

language: android
sudo: required
jdk: openjdk8

android:
  components:
    # https://github.com/travis-ci/travis-ci/issues/5036
    - tools
    # The BuildTools version used by your project
    - build-tools-28.0.3
    # The SDK version used to compile your project
    - android-28
    # Additional components
    - extra-google-m2repository
    - extra-android-m2repository

before_install:
  # Get rid of warning
  - touch $HOME/.android/repositories.cfg
  # Accept licences
  - echo yes | sdkmanager --update > /dev/null
  - echo yes | sdkmanager --licenses > /dev/null
  # Install the rest of tools (e.g., avdmanager).
  - echo yes | sdkmanager tools > /dev/null
  # Update emulator
  - echo yes | sdkmanager 'emulator' > /dev/null
  # Install the system image.
  - echo yes | sdkmanager "system-images;android-24;default;armeabi-v7a" > /dev/null
  # Create and start emulator for the script. Meant to race the install task.
  - echo no | avdmanager create avd --force -n emulatorname -k "system-images;android-24;default;armeabi-v7a"
  - $ANDROID_HOME/emulator/emulator -memory 1024 -avd emulatorname -skin 768x1280 -no-boot-anim -no-audio -no-window -no-snapshot -no-snapstorage -no-cache &

before_script:
  - echo 'org.gradle.jvmargs=-Xms4g -Xmx4g -XX:+HeapDumpOnOutOfMemoryError' >> gradle.properties
  - android-wait-for-emulator
  - adb shell settings put global window_animation_scale 0 &
  - adb shell settings put global transition_animation_scale 0 &
  - adb shell settings put global animator_duration_scale 0 &
  - adb shell input keyevent 82 &

script: ./gradlew runTests --build-cache --stacktrace

build.gradle

apply plugin: 'com.android.application'

android {

    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }

    compileSdkVersion 28
    buildToolsVersion '28.0.3'

    defaultConfig {
        applicationId "com.mypackage.foobar"
        minSdkVersion 24
        targetSdkVersion 28
        versionCode 127
        versionName "127"
        multiDexEnabled true
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }

    packagingOptions {
        exclude 'META-INF/androidx.exifinterface_exifinterface.version'
        exclude 'META-INF/proguard/androidx-annotations.pro'
    }
    
    buildTypes {
        debug {}
        release {}
    }
    testOptions {
        unitTests.returnDefaultValues = true
    }

    // Specifies flavor dimensions.
    flavorDimensions "tier" // if only one then no need to specify under each flavour

    productFlavors {
        demo {
            // dimension "tier" // optional if only one dimension
        }
    }
}

repositories {
    // ....
}

dependencies {
    // ....
     
    // unit tests
    testImplementation 'junit:junit:4.12'
    testImplementation 'pl.pragmatists:JUnitParams:1.1.1'
    testImplementation 'org.hamcrest:hamcrest-library:1.3'
    testImplementation 'org.mockito:mockito-core:2.23.0'
    testImplementation 'com.icegreen:greenmail:1.5.6'

    // instrumentation tests
    androidTestImplementation 'org.mockito:mockito-android:2.19.0'
    androidTestImplementation 'com.android.support:support-annotations:28.0.0'
    androidTestImplementation 'com.android.support.test:runner:1.0.2'
    androidTestImplementation 'com.android.support.test:rules:1.0.2'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
    androidTestImplementation 'com.android.support.test.uiautomator:uiautomator-v18:2.1.3'
}

task runTests {
    dependsOn 'testDemoDebugUnitTest', 'connectedDemoDebugAndroidTest'
}