Fix the wrong condition statement in tokio select for doh_handler and correct...

Fix the wrong condition statement in tokio select for doh_handler and correct some return code in doh_query

1. Currently, the first branch of tokio select in doh_handler is not
strict enough, which causes the first branch can never return PENDING
state if no doh_conn is available. If that happen, tokio select! might
end up comsuming all the budget and get stuck.
To fix this problem, make the precondition stricter.

2. Correct some return code in doh_query, which is used to notify
   DnsResolver fallback to DoT or UDP.

Test: manual test && atest
Bug: 196717322

Change-Id: Ib1f38f2ab7c227780213325a859e19e17fd11d3c