fix: React Navigation Instrumentation starts initial transaction before navigator mount

[jennmueng]

📢 Type of change

• Bugfix
• New feature
• Enhancement
• Refactoring

📜 Description

On V5, the initial blank transaction is now set on the routing instrumentation register instead of the navigator mount, this way it gets set on the scope before the app component is mounted. Then, once the navigator is mounted, beforeNavigate is called, and the transaction on the scope is updated with the new context.

On V4, the same strategy is adopted as V5. This means that now V4 has the option of updating a transaction on the scope instead of just setting a new one. For V4 however, this behavior is only used on the initial transaction and nothing is changed by this PR for subsequent route changes.

// Root App component

React.useEffect(() => {
    // This gets called before the next useEffect block, however, the transaction will already exist
    const transaction = Sentry.getCurrentHub().getScope().getTransaction(); // defined

    console.log(`Transaction exists: ${!!transaction}`); // "Transaction exists: true" <- Fixed from prior
}, []);

React.useEffect(() => {
    reactNavigationInstrumentation.registerNavigationContainer(navigation);
}, []);

💡 Motivation and Context

Fixes #1314

💚 How did you test it?

Updated V4 tests to account for these changes. Ran V5 on simulator, needs testing for V4.

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• All tests passing
• No breaking changes