Test: Reduce DohFrontend worker_thread to 1

Similar to aosp/1938520, we don't need to run DohFrontend on two
threads.

Also, as we don't spawn blocking tasks in DohFrontend, it doesn't
impact performance if we use the default value of max_blocking_threads.
So we remove it.

Bug: 235763745
Test: atest
Change-Id: Ifb0231f0ab6283549b3d4b38fcf89540bf0915aa