chore(compose): Update SentryTraced to emit a span only for initial composition

[0xadam-brown]

📜 Description

PR updates SentryTraced to:

1. emit a span only for initial composition, rather than for every composition of the wrapped composable; and
2. add an origin to ui.render spans, just like we already do for composition spans.

I've also cleaned up use of ImmutableHolder and renamed it to MutableRef in response to @runningcode's reasonable thoughts here.

💡 Motivation and Context

These are behavior changes to follow the fixes in #6049.

⚠️ Item (1):

• matches the current description we apply to the composition spans' parent ("Jetpack Compose Initial Composition");
• helps prevent the composition parent from getting overcrowded (see the diagram in the SentryTraced KDoc for why that's the case);
• reflects (AFAICT) the original intent of #2507; and
• is what I'd expect developers to actually care about (chime in if you think differently!).

💚 How did you test it?

I had my clanker exercise SentryTraced composables in our sample app and verify the output.

📝 Checklist

• I added GH Issue ID & Linear ID
• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• I updated the wizard if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.
• Public API changes reviewed by another Mobile SDK team member or implemented according to the develop docs spec.

🔮 Next steps

Fix the currently broken CompositionLocals that lead to the possibility of a stale parent transaction.

[sentry]

📲 Install Builds

Android

🔗 App Name	App ID	Version	Configuration
SDK Size	io.sentry.tests.size	8.55.0 (1)	release

⚙️ sentry-android Build Distribution Settings

[runningcode]

I'll give it a 👍 with the caveat that I'm not a compose expert.