How to receive notification with one signal when app is in background or closed - flutter

Question

I can only receive push notif when app is in foreground, followed instructions here and created a .java file in com.package.name now I have 2 files there MainActivity.java And NotificationServiceExtension.java
One signal version 3.4.2.
Also posted this on github
Added this on my manifest
<meta-data android:name="com.onesignal.NotificationServiceExtension"
android:value="com.bonfire.cdis.NotificationServiceExtension" />
Here is the error
Launching lib\main.dart on sdk gphone x86 64 in debug mode...
Running Gradle task 'assembleDebug'...
D:\caps\cdis\android\app\src\main\java\com\bonfire\cdis\NotificationServiceExtension.java:24: error: cannot find symbol
builder.setColor(new BigInteger("FF00FF00", 16).intValue());
^
symbol: class BigInteger
location: class NotificationServiceExtension
D:\caps\cdis\android\app\src\main\java\com\bonfire\cdis\NotificationServiceExtension.java:26: error: cannot find symbol
Spannable spannableTitle = new SpannableString(notification.getTitle());
^
symbol: class Spannable
location: class NotificationServiceExtension
D:\caps\cdis\android\app\src\main\java\com\bonfire\cdis\NotificationServiceExtension.java:26: error: cannot find symbol
Spannable spannableTitle = new SpannableString(notification.getTitle());
^
symbol: class SpannableString
location: class NotificationServiceExtension
D:\caps\cdis\android\app\src\main\java\com\bonfire\cdis\NotificationServiceExtension.java:27: error: cannot find symbol
spannableTitle.setSpan(new ForegroundColorSpan(Color.RED),0,notification.getTitle().length(),0);
^
symbol: class ForegroundColorSpan
location: class NotificationServiceExtension
D:\caps\cdis\android\app\src\main\java\com\bonfire\cdis\NotificationServiceExtension.java:27: error: cannot find symbol
spannableTitle.setSpan(new ForegroundColorSpan(Color.RED),0,notification.getTitle().length(),0);
^
symbol: variable Color
location: class NotificationServiceExtension
D:\caps\cdis\android\app\src\main\java\com\bonfire\cdis\NotificationServiceExtension.java:30: error: cannot find symbol
Spannable spannableBody = new SpannableString(notification.getBody());
^
symbol: class Spannable
location: class NotificationServiceExtension
D:\caps\cdis\android\app\src\main\java\com\bonfire\cdis\NotificationServiceExtension.java:30: error: cannot find symbol
Spannable spannableBody = new SpannableString(notification.getBody());
^
symbol: class SpannableString
location: class NotificationServiceExtension
D:\caps\cdis\android\app\src\main\java\com\bonfire\cdis\NotificationServiceExtension.java:31: error: cannot find symbol
spannableBody.setSpan(new ForegroundColorSpan(Color.BLUE),0,notification.getBody().length(),0);
^
symbol: class ForegroundColorSpan
location: class NotificationServiceExtension
D:\caps\cdis\android\app\src\main\java\com\bonfire\cdis\NotificationServiceExtension.java:31: error: cannot find symbol
spannableBody.setSpan(new ForegroundColorSpan(Color.BLUE),0,notification.getBody().length(),0);
^
symbol: variable Color
location: class NotificationServiceExtension
9 errors
FAILURE: Build failed with an exception.
* What went wrong:
Execution failed for task ':app:compileDebugJavaWithJavac'.
> Compilation failed; see the compiler error output for details.
* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.
* Get more help at https://help.gradle.org
BUILD FAILED in 7s
Exception: Gradle task assembleDebug failed with exit code 1