Google Play App Update Rejected due to SDK, which not really included

Question

When submitting an update to my app on Google Play, it is always rejecting the update saying that "We rejected App Name, with package name com.mypackage.name, for violating our Malicious Behavior or User Data policy. If you submitted an update, the previous version of your app is still available on Google Play."

Then there is this table in the email:

It seems that there automated checker sees that there is an Airpush SDK inside my app. I used that SDK before, however, it's been a long time since I removed it from my app.

I searched my project files for the term "Airpush", but I could not find any. This means that it is not on my project files anymore. What might be the reason why my app update is being constantly being rejected?

Here's my App level build.gradle:

apply plugin: 'com.android.application'
apply plugin: 'io.fabric'

android {
    compileSdkVersion 28
    buildToolsVersion '28.0.3'

    defaultConfig {
        applicationId "com.mypackage.name"
        minSdkVersion 16
        targetSdkVersion 28
        multiDexEnabled true
        versionCode 53
        versionName "5.3"
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.txt'
        }
    }
    dexOptions {
        javaMaxHeapSize "4g"
    }
}

dependencies {
    implementation fileTree(include: ['*.jar'], dir: 'libs')
    implementation 'com.android.support:appcompat-v7:26.1.0'
    compile 'com.facebook.android:audience-network-sdk:4.27.0'
    compile 'com.android.support:multidex:1.0.1'
    implementation 'com.crashlytics.sdk.android:crashlytics:2.9.6'
    implementation 'com.google.firebase:firebase-core:16.0.4'
    implementation('com.airbnb.android:lottie:2.7.0') {
        exclude group: 'com.android.support'
    }
}


apply plugin: 'com.google.gms.google-services'

And here's my Project level build.gradle:

buildscript {
    repositories {
        jcenter()
        google()
        maven {
            url 'https://maven.google.com'
        }
        maven { url 'https://plugins.gradle.org/m2/'}
        maven { url 'https://maven.fabric.io/public' }
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.2.1'
        classpath 'com.google.gms:google-services:4.0.0'
        classpath 'io.fabric.tools:gradle:1.26.1'
    }
}

allprojects {
    repositories {
        jcenter()
        google()
        maven {
            url 'https://maven.google.com/'
        }
        maven {
            url "https://jitpack.io"
        }
    }
}