This diff exposes the expo-image's `placeholder` prop ([[ https://docs.expo.dev/versions/unversioned/sdk/image/#placeholder | docs ]]). For `<LoadableImage>` and `<RemoteImage>` it is passed directly for flexibility.
For encrypted images, the thumbHash is encrypted so it must be decrypted before creating a placeholder. For non-encrypted images, it can be passed directly.
The part where `multimedia.react.js` receives the thumbhash prop will be added in next diffs whenkeyserver support is added.
Depends on D7661