…and avoid `SurfaceProducer` returning invalid `Surface` (flutter#169899)

> [!NOTE]
> For anyone reviewing this PR, see
flutter/packages#9360 for how I'd use the
`getSurface(boolean forceNewSurface)` method.

> [!NOTE]
> For anyone coming across this PR post-landing, in the code review
process, I renamed `getSurface(boolean forceNewSurface)` to
`getForcedNewSurface()`.

(1) Adds method `getSurface(boolean forceNewSurface)` to
`SurfaceProducer` that will force the creation of a new `Surface` when
`SurfaceProducer.getSurface()` is called.
(2) Fixes `SurfaceProducer` to avoid returning invalid `Surface`s when
`SurfaceProducer.getSurface()`/`getSurface(boolean forceNewSurface)` is
called.

My motivation for adding this is directly tied to
flutter#155294. The
`camera_android_camerax` plugin supports a camera preview use case that
requires providing a `Surface` to in order to render the preview. It
does so via `SurfaceProducer`; we provide a `Surface` retrieved from
`SurfaceProducer.getSurface()` to the CameraX library to render a camera
preview, and when the camera preview is done, a callback that we provide
is called, reporting that `Surface` as used and that it now should be
released/invalidated (see
[`SurfaceRequest`](https://developer.android.com/reference/androidx/camera/core/SurfaceRequest),
[`SurfaceRequest.Result`](https://developer.android.com/reference/androidx/camera/core/SurfaceRequest#provideSurface(android.view.Surface,java.util.concurrent.Executor,androidx.core.util.Consumer%3Candroidx.camera.core.SurfaceRequest.Result%3E))).
However, the CameraX library [makes no
guarantees](https://developer.android.com/reference/androidx/camera/core/Preview.SurfaceProvider#onSurfaceRequested(androidx.camera.core.SurfaceRequest):~:text=The%20camera%20may%20repeatedly%20request%20surfaces%20throughout%20usage%20of%20a%20Preview%20use%20case%2C%20but%20only%20a%20single%20request%20will%20be%20active%20at%20a%20time.)
about when requests for new `Surface`s are made, so the following race
condition can happen:

1. CameraX requests a `Surface`
2. We provide `Surface` A from `SurfaceProducer.getSurface()`
3. The camera preview was paused; CameraX requests a new `Surface`
4. The camera preview is resumed
5. We provide `Surface` A from `SurfaceProducer.getSurface()` because it
is still technically valid
6. CameraX calls our callback and now `Surface` A is released/invalid

I believe `SurfaceProducer` currently has no way to handle the level of
complexity of `Surface` handling required for use cases like the
`camera_android_camerax` camera preview, where a `Surface` may be
invalidated between calls to `SurfaceProducer.getSurface()`. So,
`getSurface(boolean forceNewSurface)` can support these use cases.*

`getSurface(boolean forceNewSurface)` simply will force
`SurfaceProducer` to return a new `Surface` when
`SurfaceProducer.getSurface` is called versus potentially returning the
same `Surface` as the previous invocation.

For `camera_android_camerax`, see
flutter/packages#9360 for an example of how it
would be used.

*I'd like to note that I also played around with creating new
`SurfaceProducer`s to solve the camera problem, which _was_ successful
and probably could be generally for that use case, but I think it's
cleaner to support this functionality from within the same
`SurfaceProducer` since we can 🤷‍♀️ It also helps avoid the user from
needing to create/manage multiple Flutter `Texture`s, from my
understanding.

forceNewSurface)` to never return an invalid `Surface`s

I noticed that `SurfaceProducer.getSurface()`/`getSurface(boolean
forceNewSurface)` does not check for `Surface` validity before returning
them while I was working on (1). Honestly, this just feels right? We
should ensure that a `Surface` is valid and can be used before returning
it and potentially confusing the user.

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above.
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Fixes] where supported.
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