Export "creating backup" string

Closes #7656

res/values/strings.xml

@@ -1376,6 +1376,7 @@
     <string name="ChatsPreferenceFragment_signal_requires_external_storage_permission_in_order_to_create_backups">Signal requires external storage permission in order to create backups, but it has been permanently denied. Please continue to app settings, select \"Permissions\" and enable \"Storage\".</string>
     <string name="ChatsPreferenceFragment_last_backup_s">Last backup: %s</string>
     <string name="ChatsPreferenceFragment_in_progress">In progress</string>
+    <string name="LocalBackupJob_creating_backup">Creating backup...</string>
     <string name="ProgressPreference_d_messages_so_far">%d messages so far</string>
     <string name="RegistrationActivity_verify_s">Verify %s</string>
     <string name="RegistrationActivity_please_enter_the_verification_code_sent_to_s">Please enter the verification code sent to %s.</string>

src/org/thoughtcrime/securesms/jobs/LocalBackupJob.java

@@ -6,6 +6,7 @@ import android.content.Context;
 import android.support.annotation.NonNull;
 import android.util.Log;
 
+import org.thoughtcrime.securesms.R;
 import org.thoughtcrime.securesms.backup.FullBackupExporter;
 import org.thoughtcrime.securesms.crypto.AttachmentSecretProvider;
 import org.thoughtcrime.securesms.database.DatabaseFactory;
@@ -46,7 +47,8 @@ public class LocalBackupJob extends ContextJob {
       throw new IOException("No external storage permission!");
     }
 
-    GenericForegroundService.startForegroundTask(context, "Creating backup");
+    GenericForegroundService.startForegroundTask(context,
+                                                 context.getString(R.string.LocalBackupJob_creating_backup));
 
     try {
       String backupPassword  = TextSecurePreferences.getBackupPassphrase(context);