Why does Android Studio think my App Engine is an Android app?

Question

I have a project containing

• Android application module
• App Engine (Cloud Endpoints)

When I created a project with an older version of Android Studio, it created a link between the modules that I deleted. Before I did this, I also tried to raise the minimum version in the application module to 21, but Android Studio or LINT somehow believed that my App Engine module was an application and marked the code, for example

• list.isEmpty()
• new AbstractMap.SimpleEntry<>()

with something like Call required API level 9 (current min is 1): (...).

The project compiles fine, but errors are underlined in red and displayed in LINT.

Neither in the gradle project nor in the gradle module do I see anything suspicious. Any idea what could be the problem or where to look and fix it?

gradle:

Projct:

// Top-level build file where you can add configuration options common to all sub-projects/modules.

buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:2.1.0'

        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

allprojects {
    repositories {
        jcenter()
    }
}

:

// https://github.com/GoogleCloudPlatform/gradle-appengine-plugin

buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath 'com.google.appengine:gradle-appengine-plugin:1.9.30'
    }
}

repositories {
    jcenter();
}

apply plugin: 'java'
apply plugin: 'war'
apply plugin: 'appengine'

dependencies {
    appengineSdk 'com.google.appengine:appengine-java-sdk:1.9.30'
    compile 'com.google.appengine:appengine-endpoints:1.9.30'
    compile 'com.google.appengine:appengine-endpoints-deps:1.9.30'
    compile 'com.google.appengine.tools:appengine-gcs-client:0.5'
    compile 'javax.servlet:servlet-api:2.5'
    compile 'com.googlecode.objectify:objectify:5.1.3'
    compile 'com.squareup.retrofit:retrofit:1.9.0'
    compile 'com.squareup.retrofit:converter-simplexml:1.9.0'
    compile 'io.jsonwebtoken:jjwt:0.6.0'
    compile 'org.slf4j:slf4j-parent:1.7.10' // required for JJWT
    compile 'org.slf4j:slf4j-nop:1.7.10' // suppress all internal logging in JJWT
}

appengine {
    downloadSdk = true
    appcfg {
        oauth2 = true
    }
    endpoints {
        getClientLibsOnBuild = true
        getDiscoveryDocsOnBuild = true
    }
}

:

apply plugin: 'com.android.application'

android {
    compileSdkVersion 23
    buildToolsVersion "23.0.3"

    defaultConfig {
        applicationId "cc.closeup.android.test"
        minSdkVersion 21
        targetSdkVersion 23
        versionCode 1
        versionName "1.0"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    compile 'com.android.support:appcompat-v7:23.4.0'
}