title: Can't build project after adding firebase dependency
body: <p>I am trying to test out FCM but after adding the firebase-messaging dependency the app can't build. I'm getting some warning along the lines of "WARNING: API 'variant.getMergeResources()' is obsolete and has been replaced with 'variant.getMergeResourcesProvider()'.
  It will be removed at the end of 2019." </p>
  
  <p>The error I'm getting is: </p>
  
  <blockquote>
    <p>"Dependencies using groupId com.android.support and androidx.* can not
    be combined but found
    IdeMavenCoordinates{myGroupId='com.android.support',
    myArtifactId='appcompat-v7', myVersion='28.0.0', myPacking='aar',
    myClassifier='null'} and
    IdeMavenCoordinates{myGroupId='androidx.slidingpanelayout',
    myArtifactId='slidingpanelayout', myVersion='1.0.0', myPacking='aar',
    myClassifier='null'}...</p>
  </blockquote>
  
  <p>on the line:</p>
  
  <pre><code>implementation 'com.android.support:appcompat-v7:28.0.0'
  </code></pre>
  
  <p>I've tried a solution that involves adding some "configurations.all" option to the gradle file but that didn't work. I've tried changing the version of firebase to an earlier one and that doesn't work either.</p>
  
  <p>This is my build that module level build.gradle file:</p>
  
  <pre><code>apply plugin: 'com.android.application'
  
  android {
      compileSdkVersion 28
      defaultConfig {
          applicationId "com.hfad.fcm"
          minSdkVersion 19
          targetSdkVersion 28
          versionCode 1
          versionName "1.0"
          testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
      }
      buildTypes {
          release {
              minifyEnabled false
              proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
          }
      }
  }
  
  dependencies {
      implementation fileTree(dir: 'libs', include: ['*.jar'])
      implementation 'com.android.support:appcompat-v7:28.0.0'
      implementation 'com.android.support.constraint:constraint-layout:1.1.3'
      testImplementation 'junit:junit:4.12'
      androidTestImplementation 'com.android.support.test:runner:1.0.2'
      androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
      implementation 'com.google.firebase:firebase-messaging:19.0.1'
  }
  
  apply plugin: 'com.google.gms.google-services'
  </code></pre>