Fix for clear call log dialog not being dismissed

This CL fixes a bug that could result in the progress dialog
that appears when clearing call logs to never be dismissed.
The cause of this bug is a race condition that causes
the original dialog fragment to be detached from the parent activity
before the progress dialog is dismissed.

If that happens, the progress dialog is never dismissed and stays
on screen forever. This CL fixes this by assigning the activity to
the dialog explicitly and retrieving it when trying to dismiss it.

Bug: 18836384
Change-Id: I5f8941e13dc7eb962b23e828cf47bc64f2b1a2aa