Test: Fix DnsResolverBinderTest.setLogSeverity to set correct default log severity

To ease flaky tests debugging, change the test to set the log
severity to INFO on userdebug builds before the test finishes.

DnsResolver will print more logs when running the tests.
This is expected because DnsResolver's default log severity
on userdebug builds is INFO (see resolv_init).

Bug: N/A
Test: atest
Change-Id: I9c6c3b56f9af35e1a535f3269d6949aa794c60f6