Automatically pick the Android sdk path using environment variables

platform/android/export/export.cpp

@@ -42,7 +42,7 @@ void register_android_exporter_types() {
 
 void register_android_exporter() {
 #ifndef ANDROID_ENABLED
-	EDITOR_DEF("export/android/android_sdk_path", "");
+	EDITOR_DEF("export/android/android_sdk_path", OS::get_singleton()->get_environment("ANDROID_SDK_ROOT"));
 	EditorSettings::get_singleton()->add_property_hint(PropertyInfo(Variant::STRING, "export/android/android_sdk_path", PROPERTY_HINT_GLOBAL_DIR));
 	EDITOR_DEF("export/android/debug_keystore", "");
 	EditorSettings::get_singleton()->add_property_hint(PropertyInfo(Variant::STRING, "export/android/debug_keystore", PROPERTY_HINT_GLOBAL_FILE, "*.keystore,*.jks"));