Make DohFrontend::clearQueries() synchronous

Sending a DNS query right after DohFrontend::clearQueries() is
commonly seen in PrivateDnsDohTest. However, it has a race issue
because frontend_stats_clear_queries() is asynchronous. Some
PrivateDnsDohTest tests are flaky due to that.

This change ensures that DohFrontend::clearQueries() returns after
the query count is reset.

Fix: 211362650
Test: Ran PrivateDnsDohTest.PreferIpv6 1000 times
Change-Id: I537c44b2015075130100e19279070c3e1ff698d3