mms: prevent FC if there isn't and associated activity

If the activity associated to span (email or browser) is not present or is disabled, then
that leads to an ActivityNotFoundException. Just graful display a message instead of crash.

JIRA: BUGDUMP-23165
https://jira.cyanogenmod.org/browse/BUGDUMP-23165
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>

Change-Id: Iaab2c2aac199f641a6df12e20bda7fecce8c2d39