[keep-awake] Add a flag to enable or disable useKeepAwake.

[macksal]

Why

useKeepAwake is a nice declarative interface for activating KeepAwake for as long as a component is on-screen. Great!

Now, let's only keep the screen awake when in a particular state, e.g. while a video or animation is playing.

const MyVideoPlayer = () => {
    const [isPlaying, setPlaying] = useState(true);
    // ❌ oops, we can't do this
    if (isPlaying) { useKeepAwake(); }
    // ...
}

We can always wrap the hook in a component to get around this, but it's not very ergonomic:

const KeepAwake = () => { useKeepAwake(); return null; }

const MyVideoPlayer = () => {
    const [isPlaying, setPlaying] = useState(true);
    // ...
    return <>
        {isPlaying && <KeepAwake />}
        <MyOtherComponents />
    </>
}

What if we could do something like this instead?

const MyVideoPlayer = () => {
    const [isPlaying, setPlaying] = useState(true);
    useKeepAwake({ enabled: isPlaying });
    // ...
}

How

Let's add a boolean option enabled to useKeepAwake that allows for the effect to be disabled. If the caller doesn't provide this option, the the effect is always active (the same as the current behaviour).

When the enabled option is exactly false, we simply don't do anything in the useEffect.

Side note

The resulting interface is still a little bit awkward since the user still needs to pass the tag parameter, which should most often be undefined:

useKeepAwake(undefined, { enabled: true });

It'd be nice to ad an overload allowing for the options object to be the first parameter. I doubt that providing tag is more frequent than providing options especially after #28884. Personally, I'd move tag inside the options parameter and deprecate the current call signature, but let's think about that later.

Test Plan

• Added unit tests to check KeepAwake is activated when enabled is true or undefined/absent, and not activated when enabled is false.
• Added a unit test to verify KeepAwake is toggled when enabled changes for subsequent re-renders.
• Confirmed the existing functionality is unchanged, as verified by the existing tests.

Checklist

• Documentation is up to date to reflect these changes (eg: https://docs.expo.dev and README.md).
• Conforms with the Documentation Writing Style Guide
• This diff will work correctly for npx expo prebuild & EAS Build (eg: updated a module plugin).