Keep activity alive in background until user doesn't kill or close app?

Can I keep Android activity alive in background, until user doesn't kill or close the application.

I want to detect user clicks in background some specific hardware button clicks, as onKeyDown is not available in service, I need to keep my activity alive in background until user kills the app.

Does android allows such behavior ?

Update :

I was able to solve this using following approach :

1] Use foreground service, Ref : https://developer.android.com/guide/components/services.html#Foreground

2] To handle media button click use below approach after Android 21 + ( Use this code in onCreate() of your foreground service ) :

mediaSession = new MediaSessionCompat(this, TAG);
        mediaSession.setFlags(MediaSessionCompat.FLAG_HANDLES_MEDIA_BUTTONS);
        mediaSession.setCallback(new MediaSessionCompat.Callback() {
            @Override
            public boolean onMediaButtonEvent(Intent mediaButtonEvent) {
                if (mediaButtonEvent.getAction().equals(Intent.ACTION_MEDIA_BUTTON)) {
                    KeyEvent event = (KeyEvent) mediaButtonEvent.getParcelableExtra(Intent.EXTRA_KEY_EVENT);
                    if (KeyEvent.KEYCODE_HEADSETHOOK == event.getKeyCode() && event.getAction() == KeyEvent.ACTION_DOWN) {
                        Log.e(TAG, "Media Button clicked");
                        handleHeadphoneClick();
                    }
                }
                return false;
            }
        });
        mediaSession.setActive(true);

Ref : https://developer.android.com/guide/topics/media-apps/mediabuttons.html