Fix export filename (again)

I managed to clobber the filename of exports with fec2f3ff94.

app/src/main/java/com/beemdevelopment/aegis/ui/PreferencesFragment.java

@@ -642,7 +642,7 @@ public class PreferencesFragment extends PreferenceFragmentCompat {
         AlertDialog.Builder builder = new AlertDialog.Builder(getActivity())
                 .setTitle(R.string.pref_export_summary)
                 .setPositiveButton(android.R.string.ok, (dialog, which) -> {
-                    String filename = checked.get() ? VaultManager.FILENAME_EXPORT : VaultManager.FILENAME_EXPORT_PLAIN;
+                    String filename = checked.get() ? VaultManager.FILENAME_PREFIX_EXPORT : VaultManager.FILENAME_PREFIX_EXPORT_PLAIN;
                     filename = new VaultBackupManager.FileInfo(filename).toString();
 
                     Intent intent = new Intent(Intent.ACTION_CREATE_DOCUMENT)

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

@@ -19,8 +19,8 @@ import java.util.UUID;
 
 public class VaultManager {
     public static final String FILENAME = "aegis.json";
-    public static final String FILENAME_EXPORT = "aegis-export.json";
-    public static final String FILENAME_EXPORT_PLAIN = "aegis-export-plain.json";
+    public static final String FILENAME_PREFIX_EXPORT = "aegis-export";
+    public static final String FILENAME_PREFIX_EXPORT_PLAIN = "aegis-export-plain";
 
     private Vault _vault;
     private VaultFileCredentials _creds;