Bug 929027 - [Messages] Multiple image sharing from Gallery to Messages r=schung

https://bugzilla.mozilla.org/show_bug.cgi?id=929027

Changes for the patch
1.Allow maximum of 5 images while sharing from  Gallery for MMS
2.To avoid the concurrent call (inorder to avoid crash),created new array, pushing all the attachments and finally calling append() by passing array.
 3.Added ThreadUI.cleanFields(true); inorder to replace the existing set of images when new set images are shared from Gallery, if the compose area already have images attached without sending.
4.Added resizedImg() inorder to make the process sequential
5.Checking containsImage flag to handle the multipart/mixed type
6.Added new test and modified the existing test cases to handle the functionality