DoH: Support Early Data

Early Data can be enabled by the flag `doh_early_data`.

As the value of the flag is cached in the DoH client, if
there are some networks existing before setting a new value to
the flag, those network will still use old flag until private
DNS setting changes.

Bug: 235763732
Test: atest
Test: atest doh_ffi_test
Test: manual test on wifi.
  1. adb shell tc qdisc add dev wlan0 root netem delay 500ms
  2. Observed the first DNS query latency on a subsequent connection:
     - when doh_early_data=0/doh_session_resumption=0, it took ~1500 ms
     - when doh_early_data=1/doh_session_resumption=1, it took ~500 ms
Change-Id: I06bc5f9aa006e357dc3ecf04a693126a55782e87