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All gms/firebase libraries must use the exact same version
specification (mixing versions can lead to runtime crashes). Found
versions 17.1.0, 17.0.4, 17.0.2, 17.0.1, 16.2.2, 16.0.4, 16.0.3,
16.0.2, 16.0.1, 16.0.0. Examples include com.google.android.gms:play-services-ads:17.1.0 and
com.google.firebase:firebase-iid:17.0.4 less... (Ctrl+F1)

There are some combinations of libraries, or tools and libraries, that
are incompatible, or can lead to bugs. One such incompatibility is
compiling with a version of the Android support libraries that is not
the latest version (or in particular, a version lower than your
targetSdkVersion.)

The Gradle dependencies are as follows:

dependencies {

implementation fileTree(dir: 'libs', include: ['*.jar'])

implementation 'com.google.firebase:firebase-core:16.0.4'

implementation 'com.google.firebase:firebase-ads:17.1.0'

implementation 'pub.devrel:easypermissions:1.3.0'

implementation 'com.android.support.constraint:constraint-layout:1.1.3'

implementation 'com.google.code.gson:gson:2.8.5'

implementation 'com.android.support:appcompat-v7:28.0.0'

implementation 'com.android.support:support-v13:28.0.0'

implementation 'com.android.support:recyclerview-v7:28.0.0'

implementation 'com.android.support:design:28.0.0'

implementation 'com.android.support:palette-v7:28.0.0'

implementation 'com.github.bumptech.glide:glide:4.8.0'

annotationProcessor 'com.github.bumptech.glide:compiler:4.8.0'

implementation 'jp.wasabeef:recyclerview-animators:2.3.0'

implementation 'com.github.apl-devs:appintro:v4.2.3'

implementation 'com.iarcuschin:simpleratingbar:0.1.5'

}

I tried using the 'play-services-ads' dependency instead of the 'firebase-ads'. Also tried adding the 'firebase-iid' dependency, but for no luck.
I am not sure if this is relevant but if I see the gradle console, I see this warning 'Warning:Failed to parse host www-proxy.idc.oracle.com'.
How to resolve the dependencies correctly?

I got the problem before. If you are already messing those versions, I will suggest you go through official's Setup again. There might have something needs to be upgraded.

Especially these 2 steps:

1.Check root gradle 's gms version.

buildscript {

  dependencies {

    classpath 'com.android.tools.build:gradle:3.2.1'

    classpath 'com.google.gms:google-services:4.2.0'

  }

}

2.Update all dependencies in app's gradle to the latest, example:

implementation 'com.google.firebase:firebase-ads:17.1.0'

implementation 'com.google.firebase:firebase-messaging:17.3.4'

implementation 'com.google.firebase:firebase-database:16.0.5'

For the latest version : Firebase Release notes