Fix #5983. Missing back button

Add back button in below activities:

* AccountSetupComposition (item "Composition defaults")
* AccountSetupOutgoing (item "Outgoing server")
* AccountSetupIncoming (item "Incoming server")

app/ui/legacy/src/main/java/com/fsck/k9/activity/setup/AccountSetupComposition.java

@@ -3,12 +3,15 @@ package com.fsck.k9.activity.setup;
 import android.app.Activity;
 import android.content.Intent;
 import android.os.Bundle;
+import android.view.MenuItem;
 import android.view.View;
 import android.widget.CompoundButton;
 import android.widget.CheckBox;
 import android.widget.EditText;
 import android.widget.LinearLayout;
 import android.widget.RadioButton;
+
+import androidx.annotation.NonNull;
 import com.fsck.k9.Account;
 import com.fsck.k9.Preferences;
 import com.fsck.k9.ui.R;
@@ -48,6 +51,10 @@ public class AccountSetupComposition extends K9Activity {
         setLayout(R.layout.account_setup_composition);
         setTitle(R.string.account_settings_composition_title);
 
+        if (getSupportActionBar() != null) {
+            getSupportActionBar().setDisplayHomeAsUpEnabled(true);
+        }
+
         /*
          * If we're being reloaded we override the original account with the one
          * we saved
@@ -101,6 +108,16 @@ public class AccountSetupComposition extends K9Activity {
         }
     }
 
+    @Override
+    public boolean onOptionsItemSelected(@NonNull MenuItem item) {
+        if (item.getItemId() == android.R.id.home) {
+            onBackPressed();
+            return true;
+        }
+
+        return super.onOptionsItemSelected(item);
+    }
+
     private void saveSettings() {
         mAccount.setEmail(mAccountEmail.getText().toString());
         mAccount.setAlwaysBcc(mAccountAlwaysBcc.getText().toString());

app/ui/legacy/src/main/java/com/fsck/k9/activity/setup/AccountSetupIncoming.java

@@ -9,6 +9,7 @@ import android.os.Bundle;
 import android.text.Editable;
 import android.text.TextWatcher;
 import android.text.method.DigitsKeyListener;
+import android.view.MenuItem;
 import android.view.View;
 import android.view.View.OnClickListener;
 import android.view.ViewGroup;
@@ -117,6 +118,10 @@ public class AccountSetupIncoming extends K9Activity implements OnClickListener
         setLayout(R.layout.account_setup_incoming);
         setTitle(R.string.account_setup_incoming_title);
 
+        if (getSupportActionBar() != null) {
+            getSupportActionBar().setDisplayHomeAsUpEnabled(true);
+        }
+        
         mUsernameView = findViewById(R.id.account_username);
         mPasswordView = findViewById(R.id.account_password);
         mClientCertificateSpinner = findViewById(R.id.account_client_certificate_spinner);
@@ -563,6 +568,16 @@ public class AccountSetupIncoming extends K9Activity implements OnClickListener
         }
     }
 
+    @Override
+    public boolean onOptionsItemSelected(@NonNull MenuItem item) {
+        if (item.getItemId() == android.R.id.home) {
+            onBackPressed();
+            return true;
+        }
+
+        return super.onOptionsItemSelected(item);
+    }
+
     protected void onNext() {
         try {
             ConnectionSecurity connectionSecurity = getSelectedSecurity();

app/ui/legacy/src/main/java/com/fsck/k9/activity/setup/AccountSetupOutgoing.java

@@ -10,6 +10,7 @@ import android.os.Bundle;
 import android.text.Editable;
 import android.text.TextWatcher;
 import android.text.method.DigitsKeyListener;
+import android.view.MenuItem;
 import android.view.View;
 import android.view.View.OnClickListener;
 import android.view.ViewGroup;
@@ -99,6 +100,10 @@ public class AccountSetupOutgoing extends K9Activity implements OnClickListener,
         setLayout(R.layout.account_setup_outgoing);
         setTitle(R.string.account_setup_outgoing_title);
 
+        if (getSupportActionBar() != null) {
+            getSupportActionBar().setDisplayHomeAsUpEnabled(true);
+        }
+
         String accountUuid = getIntent().getStringExtra(EXTRA_ACCOUNT);
         mAccount = Preferences.getPreferences(this).getAccount(accountUuid);
 
@@ -348,6 +353,16 @@ public class AccountSetupOutgoing extends K9Activity implements OnClickListener,
         validateFields();
     }
 
+    @Override
+    public boolean onOptionsItemSelected(@NonNull MenuItem item) {
+        if (item.getItemId() == android.R.id.home) {
+            onBackPressed();
+            return true;
+        }
+
+        return super.onOptionsItemSelected(item);
+    }
+
     /**
      * Shows/hides password field
      */