Change allowImagePaste to allowImagePasteForThreadID so thread can be specified and sent to JS side in imagePastedEvent

Summary: Changed prop that allows image pasting from boolean to string. This allows us to specify the threadID so the multimedia message gets sent in the correct thread when an imagePastedEvent is received on the JS side.

Test Plan: NA

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, subnub

Differential Revision: https://phabricator.ashoat.com/D618