CustomQSTile: Check for bad identifier in getTitle()

If any app that uses the system shared user id publishes a custom
QS tile it can cause SystemUI to crash because getTitle() can return
a resource ID of 0.  This patch checks the resId before returning
it and if it is 0, it returns the default.

Change-Id: Iff4d094e7556ad1eb89762ac8afb3b7078a557e5