Avoid use of ParagraphConstrains const ctor (#26392)

Usages of ParagraphConstraints (from dart:ui) whose constructor could be
const as of flutter/engine#7346 are currently marked //
ignore:prefer_const_constructors in the framework until all
Google-internal embedders have been updated to an engine version that
includes the above change. These were initially updated in engine roll
flutter/flutter#26252, but broke internal embedders.

We should re-enable use of the const constructor in those cases once
internal embedders are updated.

See: https://github.com/flutter/flutter/issues/26390