This diff exposes the expo-image's placeholder prop (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 D7761