Add getAverageLatencyMs in DnsStats

This new method returns the average DNS response time by protocol,
for example, DoT. By this method, we can know the average waiting
time for a DNS response and then utilize this information to evaluate
the performance of DoT and Do53.

Bug: 188153519
Test: cd packages/modules/DnsResolver && atest
Change-Id: I19eb25d18b1eb1ab97c7efc97d56ae9a315e4222