feat(android): Report app start reason as `app.vitals.start.reason` on standalone app start transaction (#5552)

* feat(android): Report app start reason on standalone app start transaction

Read ApplicationStartInfo.getReason() (API 35+) and attach it as
app.start.reason trace data on the standalone app.start transaction
in both the foreground and headless paths.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* feat(android): Register StandaloneAppStart SDK integration marker

Advertise that standalone app start tracing is active by adding a
StandaloneAppStart marker to the SDK metadata integrations when the
feature is enabled. Internal SDK metadata only.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* docs(changelog): Note app.start.reason is searchable in Trace Explorer

Address review feedback to mention that customers can search and group
by the app.vitals.start.reason attribute.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>