Test: Make the test TestEarlyDataFlag more robust against flag reset

The test takes around 4 seconds. It's possible that the device_config
flags are reset (which resets doh flag to 0 on Q/R/S) in the middle
of the test. Therefore, set doh flag to 1 right before resetNetwork()
to ensure the flag is 1 on the test network.

Bug: 248153420
Test: TestEarlyDataFlag pass
Change-Id: Ia839093410fcd0a737d1015527b60e65d419a52c