Avoid spawning extraneous threads in CallLogAdapter.

The call-log was leaking one or two threads every time you swiped
to it in the view-pager.  This is addressed by:
- strong synchronization of thread creation/destruction
- idempotence: reuse existing thread instead of creating one

Bug: 6086303
Change-Id: Id3de706f0961f353fe3588bac2e11e335167786a