Add support for sharing vCalendar events over MMS.

Support any ACTION_SEND intent that contains a file stream extra with
the mimetype text/x-vcalendar. This Intent can will open the
ComposeMessageActivity with a Calendar event attached.

Change-Id: I33816ceb4feebfa657f3659d5ef0d721803a056f
Merge branch 'cm-12.0' of https://github.com/CyanogenMod/android_packages_apps_Mms-caf into cm-12.0