mms: Use enter for new line if mInputType is TYPE_TEXT_VARIATION_LONG_MESSAGE

Handle the INPUT_TYPE preference as in cm 10.0:
If the "show enter key" preference is set, use enter for new line and
shift + enter for sending like the QuickMessagePopup already does.
This forward ports the corresponding section in ComposeMessageActivity code.

Change-Id: I8b9e5e66705bbe1ee2c045b7e67209323ca053bc