fix failing lints in aar_init_script.gradle (#158025)

android studio is marking those three imports as unused imports.
```gradle 
import org.gradle.api.Project
import org.gradle.api.artifacts.Configuration
import org.gradle.api.publish.maven.MavenPublication
```
the lint : 
![Capture d’écran 2024-11-01
220654](https://github.com/user-attachments/assets/a152bf80-f3b4-4a6c-bdda-abfc6503d8ea)

and this method could be converted to static :
```gradle
String getFlutterRoot(Project project) {
    if (!project.hasProperty("flutter-root")) {
        throw new GradleException("The `-Pflutter-root` flag must be specified.")
    }
    return project.property("flutter-root")
}
```

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packages/flutter_tools/gradle/aar_init_script.gradle

@@ -4,16 +4,14 @@
 // The local repository will contain the AAR and POM files.
 
 import java.nio.file.Paths
-import org.gradle.api.Project
-import org.gradle.api.artifacts.Configuration
-import org.gradle.api.publish.maven.MavenPublication
+
 
 void configureProject(Project project, String outputDir) {
     if (!project.hasProperty("android")) {
         throw new GradleException("Android property not found.")
     }
     if (!project.android.hasProperty("libraryVariants")) {
-        throw new GradleException("Can't generate AAR on a non Android library project.");
+        throw new GradleException("Can't generate AAR on a non Android library project.")
     }
 
     // Snapshot versions include the timestamp in the artifact name.
@@ -79,7 +77,7 @@ void configurePlugin(Project project, String outputDir) {
     configureProject(project, outputDir)
 }
 
-String getFlutterRoot(Project project) {
+static String getFlutterRoot(Project project) {
     if (!project.hasProperty("flutter-root")) {
         throw new GradleException("The `-Pflutter-root` flag must be specified.")
     }