Crashlytics Gradle Plugin: support Gradle isolated projects with enabled option nativeSymbolUploadEnabled

### [READ] Step 1: Are you in the right place?

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### [REQUIRED] Step 2: Describe your environment

- Android Studio version: not related to the bug
- Firebase Component: Crashlytics Gradle plugin
- Component version: 3.0.7

### [REQUIRED] Step 3: Describe the problem

#### Steps to reproduce:

This issue is related to issue #6748. Unfortunately, the previous ticket did not fix the errors when using the option nativeSymbolUploadEnabled=true

#### Relevant Code:

```
org.gradle.api.InvalidUserCodeException: Project ':app' cannot dynamically look up a property in the parent project ':'

...
at org.gradle.api.internal.project.DefaultProject.hasProperty(DefaultProject.java:1176)

at com.google.firebase.crashlytics.buildtools.gradle.tasks.GenerateSymbolFileTask$Companion.register$lambda$0(GenerateSymbolFileTask.kt:177)
at com.google.firebase.crashlytics.buildtools.gradle.tasks.GenerateSymbolFileTask$Companion$inlined$sam$i$org_gradle_api_Action$0.execute(TaskContainerExtensions.kt)
```
and
```
org.gradle.api.InvalidUserCodeException: Project ':app' cannot dynamically look up a property in the parent project ':'
...
at org.gradle.api.internal.project.DefaultProject.hasProperty(DefaultProject.java:1176)

at com.google.firebase.crashlytics.buildtools.gradle.tasks.GenerateSymbolFileTask.validateSymbolGeneratorType(GenerateSymbolFileTask.kt:138)
at com.google.firebase.crashlytics.buildtools.gradle.tasks.GenerateSymbolFileTask.access$validateSymbolGeneratorType(GenerateSymbolFileTask.kt:53)
at com.google.firebase.crashlytics.buildtools.gradle.tasks.GenerateSymbolFileTask$Companion.register$lambda$0(GenerateSymbolFileTask.kt:191)
```