fix(build): add explicit Kotlin imports for Properties/FileInputStream and fix signing config casts

- Add import statements for java.util.Properties and java.io.FileInputStream
  (newer Kotlin DSL requires explicit imports)
- Replace deprecated jvmTarget = JavaVersion.VERSION_17.toString() with "17"
- Replace unsafe 'as String' casts with Properties.getProperty() in signing config
- Fix same import issue in settings.gradle.kts

android/settings.gradle.kts

@@ -1,7 +1,9 @@
+import java.util.Properties
+
 pluginManagement {
     val flutterSdkPath =
         run {
-            val properties = java.util.Properties()
+            val properties = 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" }