[expo-av] Add return type to loadAsync()

[awinograd]

NOTE/EDIT

I just realized this was happening because in SDK 36, the PlaybackStatus type wasnt exported so i was importing directly from expo-av/src/AV. If I switch to import from the top level package expo-av then the compliation error goes away. That said, I believe this code change is still correct

Why

In my project I use typescript's noImplicitAny: true option. It causes loadAsync to show as a typechecking error.

node_modules/expo-av/src/AV.ts:227:3 - error TS7010: 'loadAsync', which lacks return-type annotation, implicitly has an 'any' return type.                    
                                                                                                                                                              
227   loadAsync(source: AVPlaybackSource, initialStatus: AVPlaybackStatusToSet, downloadAsync: boolean);                                                      
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 

How

The docs say that loadAsync returns a Promise. https://docs.expo.io/versions/v37.0.0/sdk/av/#playback-api

Test Plan

If I use this patch locally, the compilation error goes away

[bbarthec]

Hello @awinograd 👋 Thank you for your contribution! 🎉
To have your contribution fully mergable I need you to do the followings:

• run yarn & yarn build in packages/expo-av to regenerate .d.ts files according to your changes
• add entry in packages/expo-av/CHANGELOG.md according to the style that you can find in main CHANGELOG.md in repository 😉

[bbarthec]

@awinograd, sth went really bad on CI, cause it cannot access your repo 😞🤔
May I ask you to rebase on master and we'll see if the problem persists. If so, I would take over and fix it on my own 😉

[awinograd]

@bbarthec I originally did a shallow clone of the repo so perhaps that was the issue? Anyway, I just rebased

[bbarthec]

@awinograd, everything seems almost perfect, but yarn lint failed on expo-av package now 😞
I need you to run yarn lint in expo-av and fix the error you'll get.
That should make required sdk CircleCI pipeline togo green 🎉

[bbarthec]

Replaced with #7704 as CI errors prevents from merging it from here 😞