Bundle source code context lines with Android stack frames

[ioannisj]

Extracted from PostHog/posthog#43012.

Problem

Captured frames carry only module, function, lineno, and filename (ThrowableCoercer.kt builds them without any surrounding source), so the error UI can't show the lines around a frame.

Scope

• Emit pre_context, context_line, and post_context for in-app frames.
• Bundle and upload the source from the Gradle plugin, teach posthog-cli to carry it, and render it on the backend.

Components: SDK, Gradle plugin, posthog-cli, backend. iOS and Flutter-iOS already have source context.