Fix DialtactsActivity leaks

The CallLogFragment attaches to ViewPager, and does not show by default,
so it still stay in pause status. If straight exiting Activity, the
method onPause is not called so that the query thread cannot be interrupted.
The class QueryThread is inner class that holds the instance of ContactInfoCache.
But the member mContactInfoHelper of ContactInfoCache already holds the context
of DialtactsActivity so that the instance of DialtactsActivity cannot be recycled.
In order to avoid it, must stop the query thread in the method onStop.

Change-Id: I0bbbf25b95ae82a2d67152edd07c4f7c14498917
CRs-Fixed: 1083453