This is one of a series of changes to reland https://github.com/flutter/engine/pull/47239.
This PR changes `Animator` so that if `Render` is not called after a `BeginFrame`, this call is ignored.
Note that this is slightly different from https://github.com/flutter/engine/pull/47239. Instead of saying that we should ultimately change this skip to an assertion, this PR aims to keep the skip as the final shape. This is because a while ago we (with @goderbauer and @loic-sharma) decided that `PlatformDispatcher` should contain as little logic as possible to allow testing, and instead serve as a minimal native function binding, which means that we should eventually move the code that validates calling convention to the engine.
[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
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