Fix Android document opening silently failing (PDF and others)

[Copilot]

On Android, calling openDocument() silently did nothing — no error, no viewer launched — while iOS worked fine. Three bugs combined to cause this.

Root causes & fixes

• ClassCastException killing the activity reference (OpenDocumentPlugin.kt): binding.activity as FlutterActivity throws when the host app uses FlutterFragmentActivity (standard modern Flutter setup), leaving activity = null. The null-safe ?.startActivity(intent) then silently no-ops. Fixed by using android.app.Activity throughout — no cast needed since ActivityPluginBinding.getActivity() already returns Activity.

  // Before — crashes silently for FlutterFragmentActivity hosts
  it?.setActivity(binding.activity asFlutterActivity)
  // After
  it?.setActivity(binding.activity)

• result.success() never called (OpenDocument.kt): After startActivity, neither result.success() nor result.error() was invoked on the happy path, so the Dart await would never resolve.

• Wrong file extension for multi-dot filenames: name(url).split(".")[1] picks index 1, so my.report.pdf → MIME type for report instead of pdf. Changed to substringAfterLast(".").

• FLAG_ACTIVITY_SINGLE_TOP → FLAG_ACTIVITY_NEW_TASK: Ensures the viewer launches correctly when called from a background thread.