chore: support build as a flutter module

When build as a flutter module and integrated into the existing android project,
there is no applicationVariants and result in following compilation failure:

Could not get unknown property 'applicationVariants' for extension 'android' of
type com.android.build.gradle.LibraryExtension.

In such case, we will use libraryVariants instead

gradle/plugin.gradle

@@ -116,8 +116,12 @@ class CargoKitPlugin implements Plugin<Project> {
         }
 
         def cargoBuildDir = "${project.buildDir}/build"
+
+        // Determine if the project is an application or library
+        def isApplication = plugin.project.plugins.hasPlugin('com.android.application')
+        def variants = isApplication ? plugin.project.android.applicationVariants : plugin.project.android.libraryVariants
 
-        plugin.project.android.applicationVariants.all { variant ->
+        variants.all { variant ->
 
             final buildType = variant.buildType.name