title: Firebase crashlytics fails build after updating android gradle plugin to 7.0.2
body: <p>I'm working on multi-flavor android app
  After updating the Android Gradle plugin to the <code>7.0.2</code> version (from '4.1.1') to access a new variant API, I get the following error during the app build:</p>
  <pre><code>FAILURE: Build failed with an exception.
  
  * What went wrong:
  Some problems were found with the configuration of task ':app:injectCrashlyticsMappingFileIdRandstadAccDebug' (type 'InjectMappingFileIdTask').
  - Type 'com.google.firebase.crashlytics.buildtools.gradle.tasks.InjectMappingFileIdTask' property 'mappingFileId' of mutable type 'org.gradle.api.provider.Property' is writable.
  
  Reason: Properties of type 'org.gradle.api.provider.Property' are already mutable.
  
  Possible solution: Remove the 'setMappingFileId' method.
  
  Please refer to https://docs.gradle.org/7.0.2/userguide/validation_problems.html#mutable_type_with_setter for more details about this problem.
  - Type 'com.google.firebase.crashlytics.buildtools.gradle.tasks.InjectMappingFileIdTask' property 'resourceFile' of mutable type 'org.gradle.api.file.RegularFileProperty' is writable.
  
  Reason: Properties of type 'org.gradle.api.file.RegularFileProperty' are already mutable.
  
  Possible solution: Remove the 'setResourceFile' method.
  
  Please refer to https://docs.gradle.org/7.0.2/userguide/validation_problems.html#mutable_type_with_setter for more details about this problem.
  
  * Try:
  Run with --stacktrace option to get the stack trace.  Run with --scan to get full insights.
  
  * Get more help at https://help.gradle.org
  </code></pre>
  <p>I'm using the latest version of firebase BOM 28.4.1.</p>
  <p>Is there any walkaround for this?</p>