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

Updated review comments
Follow-up:

* do not show max size alert for images
* changed slightly the tests
Follow-ups:

* change the string when there is several files
* remove useless Mock_alert
* indentation changes
Merge pull request #17658 from julienw/929027-multiple-share

Bug 929027 - multiple share, r=schung
Revert "Bug 991088 - Fix test_system_notification_bar.py" after being pinged in #gaia for travis red

This reverts commit 393dca9ca5fcf761a49b0dc58aeb59057b83992e.
Bug 937442 - (lockscreen-window) [Window Management] Implement LockscreenWindow to manage LockScreen.