DoH: Reduce DOH_THREADS from 2 to 1

Make the doh engine to run on one single thread. This change
doesn't seem to improve DoH query latency, but it does save
some CPU resource. See the bug for the test result.

Bug: 213438941
Test: cd packages/modules/DnsResolver && atest
Change-Id: I9c33e10d9269ba14c9ce708d0bc4b3761f908dea