Bug 974733 - [Sora][Message] We should not change panels when handling the "sending" event on the window that does not send the message r=schung

When the user presses the "send" button, we set a boolean, so
that when we receive the "sending" event, we know whether we're in the same
window.

(cherry picked from commit a8c1a5ad0936cf5c131afaf2bb5cf6b068e8535d)

Conflicts:
	apps/sms/js/thread_ui.js
	apps/sms/test/unit/thread_ui_test.js
Merge pull request #16745 from julienw/974733-v1.3-sending-sms-activity

Bug 974733 - [Sora][Message]The conversation message display wrong after...