WAL2-85 Fix crash on validator setup flow

[Radiokot]

Purpose

The crash was caused by missing Serializable marker on SimpleFraction, introduced with WC cost estimation fix.

Changes

• Add missing Serializable marker on SimpleFraction

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.

[linear]

WAL2-85 CryptoX-Android App is crashing while adding new validator

To reproduce:

1. Recover from seed phrase on Testnet.
2. Add validator with stake 10000 CCDs.

Result:

App crashes before proceeding to commission rates screen.

Expected:

After setting commission rates, user can add validator.

Environment:

0.7.0-qa.8 - Android 11

add-validator.mp4