Drop API 28 and 29 from instrumentation test matrix

Based on timing analysis across two full CI runs:
- API 28 averaged 652s (~11 min), slowest after API 29
- API 29 averaged 672s (~11 min), consistently the slowest new addition

API 31, 33, and 34 provide good modern Android coverage at reasonable
cost (568–622s avg). APIs 28 and 29 can be reconsidered later if
Android 9/10 coverage becomes a specific need.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>