Make PrivateDnsDohTest.SessionResumption more robust against flag reset

This test usually takes 7 seconds to finish, and resetNetwork() is
called during the test. If the flag "doh" is set to 0 during the
test, the test will fail. Therefore, this change adds another
ScopedSystemProperties for "doh" right before resetNetwork().

Bug: 222023286
Test: ran PrivateDnsDohTest.SessionResumption
Change-Id: I4831cfa4324b5970db0f281afacba8141929520c