Support Gradle Isolated Projects feature

Support Gradle Isolated Projects feature in secrets-gradle-plugin. Currently using this plugin with [`-Dorg.gradle.unsafe.isolated-projects=true`](https://docs.gradle.org/current/userguide/isolated_projects.html) fails due to [project.rootProject.loadPropertiesFile(it)](https://github.com/google/secrets-gradle-plugin/blob/main/secrets-gradle-plugin/src/main/java/com/google/android/libraries/mapsplatform/secrets_gradle_plugin/SecretsPlugin.kt#L47) as it reaches out to a the root project and that is not safe when all projects are configured at the same time.

Given the only reason rootProject is used is to get the root project directory, it should be easy to move to using [`Project.getRootDir`](https://docs.gradle.org/nightly/javadoc/org/gradle/api/Project.html#getRootDir()) that is Isolated Projects safe.