DIA-6123 prevent messageUI from being created on finished()

[andresilveirah]

• disposes messageUI view before setting its reference to null

• Fixes a race condition causing the onUIReady to be called more than once while the consent ui is still being presented. More info below.

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1. First layer message loads, onUIReady() is called (isPresenting: false → true) ✅
2. User takes a consent action (accept/reject all or save & exit)
3. Async coroutine starts but finished(view) is called immediately
4. If timing is right and the async coroutine executes fast enough, it sets messageUI = null
5. Then finished() calls getOrCreateMessageUI().isPresenting = false
6. If messageUI is already null, getOrCreateMessageUI() creates a NEW WebView instance
7. The NEW WebView has isPresenting = false by default
8. renderNextMessageIfAny() or loadPrivacyManager() then calls getOrCreateMessageUI().load()
9. This uses the newly created WebView (or creates another one)
10. When content loads, JavaScript calls loaded()
11. Guard checks: isPresenting is false (new instance), so it calls onUIReady() again ❌