How to capture key events inside a service?

A Service has no UI so it doesn't receive any input from the User.

Now if you had an activity that managed the service then you could make the service do something special when the back key was pressed while in your activity.

You can capture hardware key button events by using accessibility service, only you need to enable the service by going to accessibility settings after installing the app. Here is the code

    public class AccessiblityService extends AccessibilityService {

        @Override
        protected void onServiceConnected() {
            super.onServiceConnected();
            Log.d(TAG, "service is connected");
        }

        @Override
        public void onAccessibilityEvent(AccessibilityEvent accessibilityEvent) {

            Log.d(TAG, "onAccessibiltyEvent" + accessibilityEvent.toString());

        }

        @Override
        public void onInterrupt() {

        }
    // here you can intercept the keyevent
        @Override
        protected boolean onKeyEvent(KeyEvent event) {
            return handleKeyEvent(event);
        }

         private boolean handleKeyEvent(KeyEvent event) {
            int action = event.getAction();
            int keyCode = event.getKeyCode();
            if (action == KeyEvent.ACTION_DOWN) {
                switch (keyCode) {
                    case KeyEvent.KEYCODE_VOLUME_DOWN:
                        //do something
                        return true;

                    case KeyEvent.KEYCODE_VOLUME_UP: {
                        //do something
                        return true;
                    }
                }
            }
            return false;
        }

} 

manifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    package="com.mypackagename">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme"
        tools:ignore="GoogleAppIndexingWarning">
        <service
            android:name=".AccessiblityService"
            android:permission="android.permission.BIND_ACCESSIBILITY_SERVICE">
            <intent-filter>
                <action android:name="android.accessibilityservice.AccessibilityService" />
            </intent-filter>
            <meta-data
                android:name="android.accessibilityservice"
                android:resource="@xml/accessibility_service" />
        </service>
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>             
    </application>

</manifest>

accessibility_service.xml //create it in "xml" directory

<?xml version="1.0" encoding="utf-8"?>
<accessibility-service xmlns:android="http://schemas.android.com/apk/res/android"
    android:canRequestFilterKeyEvents="true"
    android:accessibilityFlags="flagRequestFilterKeyEvents"/>