[v10] Revisit app-start span names

[buenaflor]

Context

Part of the Major v10 Task Collection.

Problem

Some app-start span descriptions are opaque from a user perspective — e.g. the "SentrySetup" phase renders as "Before Sentry Init Setup".

Expected outcome

• App-start span descriptions renamed to clear, user-facing terms.
• Naming consistent across handler v1 and v2.

Pointers

• packages/flutter/lib/src/integrations/native_app_start_data.dart:44-50
• packages/flutter/lib/src/integrations/native_app_start_handler.dart
• packages/flutter/lib/src/integrations/native_app_start_handler_v2.dart

Breaking change: No (span descriptions) — note it may affect users' saved searches/dashboards.

[linear-code]

DART-424