Create MMS - Selection of several images at once doesn't work

Mako Utopic #122

The specification of the communication hub [¹] indicates that multi-selection of images is possible when the user creates an MMS but it is not.

TEST CASE
0. Verify that several images are available in the gallery app
1. Launch the messaging app and create a new message
2. Tap the 'camera' icon to launch the gallery app
3. Long press a picture to select it
4. Long press another picture to select it

ACTUAL RESULT
On step 4, the second picture is selected by the first picture is deselected.

EXPECTED RESULT
Both pictures are selected.

[¹] https://docs.google.com/a/canonical.com/document/d/1BRy9-DEXr3A1D7PHkJaXmjbe7xbRmxKkzANXuu-F6lc/edit#heading=h.yp4wp73l2r0c