[native] await sendMultimediaMessage in KeyboardInputHost.onMediaGalleryItemSelected

Summary:
We could add a void here instead, but onMediaGalleryItemSelected is already an async function, and the place we're passing it to doesn't await the result anyways. So might as well just add an await keyword.

Depends on D10170

Test Plan: Flow

Reviewers: michal, tomek

Reviewed By: michal

Differential Revision: https://phab.comm.dev/D10171