Bug 1707719 - Avoid `groovy.lang.MissingMethodException ... getFileNames()`. r=geckoview-reviewers,agi

The exact error is
```
groovy.lang.MissingMethodException: No signature of method: groovy.util.FileNameFinder.getFileNames() is applicable for argument types: (File, String)
```

Differential Revision: https://phabricator.services.mozilla.com/D140689

mobile/android/gradle/with_gecko_binaries.gradle

@@ -64,7 +64,7 @@ def getLlvmBinFolder() {
     // and return its parent directory.
     return project
         .file(new FileNameFinder()
-                  .getFileNames(ndkDirectory, "toolchains/llvm/prebuilt/**/bin/llvm-*")
+                  .getFileNames(ndkDirectory.toString(), "toolchains/llvm/prebuilt/**/bin/llvm-*")
                   .first())
         .parentFile
 }