Don't let old Launcher activity interfere with new one

-> Launcher uses a static instance of it's loader (across
   multiple activities) since activities can come and go
   (configuration change, eg.) but the data model and loading
   is static.
-> Currently, this is not robust to a sequence of events that
   looks like onCreate(instance A), onCreate(instance B),
   onDestroy(instance B) -- depending on the timing of those
   calls.
-> This CL addresses a symptom of the above scenario by not
   allowing an older Launcher Activity cancel the loader /
   clear the callbacks for a newer Activity.

Bug 17679693

Change-Id: I8ece93e288464b0d578b9669c165b67132d997ed