Fix settings crash on users page on config change

The background task loses the reference to the Activity once it is
destroyed and recreated after a config change (such as screen rotation).
This resulted in getContext() returning null and openUserDetails()
crashing.
To fix this, we simply store the context before dispatching the
background worker.

Bug: 256021119
Test: manually tested
Change-Id: I3399fa5586de0cb5fc159204166f92902dd42de0