[Test] Deflake ResolverTest#GetAddrInfoV4_deferred_resp

There are two threads in the test that will set different DNS servers
and send queries to the DNS resolver. It relies on a 100ms sleep between
the two threads. When the problem occurs, the work of the first thread
is somehow delayed, causing the second thread's DNS query to be sent to
the DNS server configured by the first thread.

Expected behavior:
1. Thread 1 create.
2. Thread 1 set DNS server to 127.0.0.10.
3. Thread 1 send DNS query to 127.0.0.10.
4. Thread 2 create.
5. Thread 2 set DNS server to 127.0.0.11.
6. Thread 2 send DNS query to 127.0.0.11.

Failed case:
1. Thread 1 create (and then being delayed for over than 100ms)
2. Thread 2 create.
3. Thread 2 set DNS server to 127.0.0.11.
4. Thread 1 set DNS server to 127.0.0.10.
5. Thread 2 send DNS query to 127.0.0.10. <-- NG
6. Thread 1 send DNS query to 127.0.0.10.

Replacing unreliable sleep by a boolean flag.

Bug: 303371206
Bug: 302623174
Test: atest resolv_integration_test
Change-Id: I56222894a8600c7cb4c1911dd453756d6069bd02