[native] Disable ImagePasteModal send button after first press

Summary:
Disable the button after it's pressed for the first time to prevent issue with multiple sends.

Also moved inputState.sendMultimediaMessage(selection, threadInfo); to an IIFE.

Test Plan:

1. Copy image to clipboard
2. Paste image into TextInput to pull up ImagePasteModal
3. Try hitting the send button in quick succession
4. Make sure that the image only gets sent once

Reviewers: palys-swm, def-au1t, varun, benschac, ashoat

Reviewed By: ashoat

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3424