docs: record Android 8 emulator compatibility

[zefutoff]

Summary

• document Android 8.0 / API 26 AVD compatibility
• record the exact emulator and active WebView provider
• record SpellCheckerService, PROCESS_TEXT and IME behavior
• clarify that emulator coverage does not replace physical-device validation

Validation

• make check
• make assemble
• :spellchecker:connectedDebugAndroidTest on API 26
• :app:connectedDebugAndroidTest on API 26
• manual WebView compatibility smoke test on API 26

Manual API 26 result

• SpellCheckerService: no native WebView feedback
• PROCESS_TEXT: action available, returned correction not applied by WebView
• IME correction: works
• late selection while IME is already open: works
• previous keyboard fallback: Android input-method picker on API 26

The roadmap item requiring older physical-device validation remains open.