feat: enable captureApplicationLifecycleEvents by default and align Android config key

[marandaneto]

💡 Motivation and Context

captureApplicationLifecycleEvents was disabled by default, requiring explicit opt-in. This changes the default to true so lifecycle events are captured out of the box.

Additionally, the Android manifest config key is aligned with iOS:

• New key: com.posthog.posthog.CAPTURE_APPLICATION_LIFECYCLE_EVENTS
• Legacy key: com.posthog.posthog.TRACK_APPLICATION_LIFECYCLE_EVENTS (still supported for backward compatibility)

The new key is checked first; if not present, the legacy key is used. If neither is set, defaults to true.

The changeset includes instructions on how to disable lifecycle events for users who don't want them.

💚 How did you test it?

Manual verification of manifest key resolution logic and default values on Android and iOS.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran pnpm changeset to generate a changeset file
• Added the release label to the PR

[marandaneto]

android and ios are enabled by default, so i dont see a reason why not to enable by default on flutter as well