Investigate Android snapshot helper secondary-display support

## Context

#455 originally tracked several Android snapshot helper follow-ups. PR #470 covers the install-result cache and release-mode helper smoke, so secondary-display support should be tracked separately.

## Goal

Investigate whether the Android snapshot helper should support capturing UI from non-default displays via Android multi-display APIs.

## Notes

- Keep this driven by a concrete provider/device scenario rather than adding broad display routing speculatively.
- Preserve the existing helper-first snapshot flow and stock UIAutomator fallback behavior.
- Consider how display selection would be exposed through CLI/client options before changing helper internals.
- Add coverage only when the test environment can exercise or meaningfully simulate secondary displays.

## Prior art

Split out from #455 after #470 covered the immediate reliability/performance items.