Always copy VaultFileCredentials when accessing it

This makes sure that mutations to the credentails can only ever be persisted
intentionally.

app/src/main/java/com/beemdevelopment/aegis/ui/fragments/preferences/SecurityPreferencesFragment.java

@@ -241,10 +241,12 @@ public class SecurityPreferencesFragment extends PreferencesFragment {
             if (!isBackupPasswordSet()) {
                 Dialogs.showSetPasswordDialog(requireActivity(), new SetBackupPasswordListener());
             } else {
-                SlotList slots = _vaultManager.getVault().getCredentials().getSlots();
+                VaultFileCredentials creds = _vaultManager.getVault().getCredentials();
+                SlotList slots = creds.getSlots();
                 for (Slot slot : slots.findBackupPasswordSlots()) {
                     slots.remove(slot);
                 }
+                _vaultManager.getVault().setCredentials(creds);
 
                 saveAndBackupVault();
                 updateEncryptionPreferences();

app/src/main/java/com/beemdevelopment/aegis/vault/VaultFileCredentials.java

@@ -1,9 +1,12 @@
 package com.beemdevelopment.aegis.vault;
 
+import androidx.annotation.NonNull;
+
 import com.beemdevelopment.aegis.crypto.CryptParameters;
 import com.beemdevelopment.aegis.crypto.CryptResult;
 import com.beemdevelopment.aegis.crypto.MasterKey;
 import com.beemdevelopment.aegis.crypto.MasterKeyException;
+import com.beemdevelopment.aegis.util.Cloner;
 import com.beemdevelopment.aegis.vault.slots.SlotList;
 
 import java.io.Serializable;
@@ -45,4 +48,10 @@ public class VaultFileCredentials implements Serializable {
     public VaultFileCredentials exportable() {
         return new VaultFileCredentials(_key, _slots.exportable());
     }
+
+    @NonNull
+    @Override
+    public VaultFileCredentials clone() {
+        return Cloner.clone(this);
+    }
 }

app/src/main/java/com/beemdevelopment/aegis/vault/VaultRepository.java

@@ -220,11 +220,11 @@ public class VaultRepository {
     }
 
     public VaultFileCredentials getCredentials() {
-        return _creds;
+        return _creds == null ? null : _creds.clone();
     }
 
     public void setCredentials(VaultFileCredentials creds) {
-        _creds = creds;
+        _creds = creds == null ? null : creds.clone();
     }
 
     public boolean isEncryptionEnabled() {