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

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

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

Conflicts:
	res/values/custom_strings.xml

Change-Id: Iaab2c2aac199f641a6df12e20bda7fecce8c2d39