Deprecation of deferFirstFrameReport and allowFirstFrameReport.

[Michael Goderbauer]

Hello!

In https://github.com/flutter/flutter/pull/45135 I would like to depreciate WidgetsBinding.deferFirstFrameReport and WidgetsBinding.allowFirstFrameReport. Instead, I am introducing RendererBinding.deferFristFrame and RendererBinding.allowFirstFrame.

The new API will actually delay sending frames to the engine, whereas the old API would just omit reporting metrics for the first frames.

As it turns out, delaying the entire frame is actually more useful as this will keep the splash screen up longer and avoid showing an incompletely rendered app to the user.

The old API will be around for a couple of releases. To migrate, just call deferFristFrame/allowFirstFrame instead of deferFirstFrameReport/allowFirstFrameReport.

Please comment on the PR if you have any concerns.

Michael