fix(build): revert settings.gradle.kts to fully-qualified java.util.Properties

The settings.gradle.kts pluginManagement block runs in a different
context where top-level imports are not available. Use fully-qualified
java.util.Properties() here while keeping explicit imports in
app/build.gradle.kts where they are required.

android/settings.gradle.kts

@@ -1,9 +1,7 @@
-import java.util.Properties
-
 pluginManagement {
     val flutterSdkPath =
         run {
-            val properties = Properties()
+            val properties = java.util.Properties()
             file("local.properties").inputStream().use { properties.load(it) }
             val flutterSdkPath = properties.getProperty("flutter.sdk")
             require(flutterSdkPath != null) { "flutter.sdk not set in local.properties" }