TrustedCredentials: Don't crash when the activity is paused

If you press Settings -> Security -> Trusted Credentials, then press
 back before the loading finishes, the loader async task will still
try to dereference the activity which will cause an NPE.

This patch fixes that by getting and holding a reference to the
context in the beggining and bailing early if it is null.

Change-Id: Ie03734d5f184821df5d8729fc4a16eb51cdeebf5