[native] Create real thread when image is sent by camera modal

Test Plan:
Send an image to an existing thread: it should not create a new thread.
Open a pending sidebar, send an image and verify that the thread was created.

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, atul, subnub, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D720