Fix testing DoH server not to concatenate responses

This comes from one of the review comments in aosp/1881165 that
points out a potential bug in testing DoH server. Instead of
concatenating all the packets returned by conn.send(), we should
send each packet separately.

Besides, flush_egress() is changed not to return Result::Ok if
conn.send() returns quiche::Error::Done.

Bug: 181642979
Test: cd packages/modules/DnsResolver && atest
Change-Id: I667f2f15f9db399a8362fc2be28f89c5bd3a016a