Read properties files as UTF-8 in Gradle scripts (#13981)

Fixes https://github.com/flutter/flutter/issues/13972

packages/flutter_tools/gradle/flutter.gradle

@@ -46,7 +46,7 @@ class FlutterPlugin implements Plugin<Project> {
     private Properties readPropertiesIfExist(File propertiesFile) {
         Properties result = new Properties()
         if (propertiesFile.exists()) {
-            propertiesFile.withInputStream { stream -> result.load(stream) }
+            propertiesFile.withReader('UTF-8') { reader -> result.load(reader) }
         }
         return result
     }

packages/flutter_tools/templates/create/android-java.tmpl/app/build.gradle.tmpl

 def localProperties = new Properties()
 def localPropertiesFile = rootProject.file('local.properties')
 if (localPropertiesFile.exists()) {
-    localPropertiesFile.withInputStream { stream ->
-        localProperties.load(stream)
+    localPropertiesFile.withReader('UTF-8') { reader ->
+        localProperties.load(reader)
     }
 }
 

packages/flutter_tools/templates/create/android-kotlin.tmpl/app/build.gradle.tmpl

 def localProperties = new Properties()
 def localPropertiesFile = rootProject.file('local.properties')
 if (localPropertiesFile.exists()) {
-    localPropertiesFile.withInputStream { stream ->
-        localProperties.load(stream)
+    localPropertiesFile.withReader('UTF-8') { reader ->
+        localProperties.load(reader)
     }
 }
 

packages/flutter_tools/templates/create/android.tmpl/settings.gradle

@@ -5,7 +5,7 @@ def flutterProjectRoot = rootProject.projectDir.parentFile.toPath()
 def plugins = new Properties()
 def pluginsFile = new File(flutterProjectRoot.toFile(), '.flutter-plugins')
 if (pluginsFile.exists()) {
-    pluginsFile.withInputStream { stream -> plugins.load(stream) }
+    pluginsFile.withReader('UTF-8') { reader -> plugins.load(reader) }
 }
 
 plugins.each { name, path ->