Permanently Enabling a Button when another Button is Clicked. Even the app is restarted

I Have 2 Buttons Btn1 & Btn2. Btn2 is Disabled But when Btn1 is Clicked Btn2 Should be Enabled Permanently even the app is Restarted

Button Btn1,Btn2;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    Btn1 = (Button)findViewById(R.id.EnabledBtn);
    Btn2 = (Button)findViewById(R.id.disbledBtn);

    Btn1.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            SharedPreferences pref = PreferenceManager.getDefaultSharedPreferences(MainActivity.this);
            pref.edit().putBoolean("isEnabled",false).apply();
            Btn2.setEnabled(true);
        }
    });
}

@Override
protected void onResume() {
    super.onResume();
    SharedPreferences pref = PreferenceManager.getDefaultSharedPreferences(MainActivity.this);
    boolean enabled = pref.getBoolean("isEnabled",true);
    Btn2.setEnabled(enabled);


}

}

Change your code to be

SharedPreferences pref;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    pref = PreferenceManager.getDefaultSharedPreferences(MainActivity.this);

    Btn1 = (Button)findViewById(R.id.EnabledBtn);
    Btn2 = (Button)findViewById(R.id.disbledBtn);

  /*when the app is launched, it will check if the button should be 
    enabled by getting the value from sharedpreferences. If the value was 
    not already set, then it chooses false as the default value*/

    boolean enabled = pref.getBoolean("isEnabled",false);
    Btn2.setEnabled(enabled);

  /*When btn1 is clicked, you want to store value true to 
    sharedpreferences so that it will be the value that will be 
    retrieved when app is restarted*/

    Btn1.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            pref.edit().putBoolean("isEnabled",true).apply();
            Btn2.setEnabled(true);
        }
    });
}

Please check the comments inside the code to understand why the changes need to the made.