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.
Merge pull request #16798 from julienw/974733-sending-sms-activity-v2

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