[DPP] Use Executor instead of Handler in Easy Connect API

Following API Council review, use Executor instead of Handler
in all Easy Connect API.

Bug: 122984003
Test: atest com.google.android.gts.setupwizard.DppHostTest
Test: atest DppManagerTest
Test: act.py -c ../WifiDppConfig.json -tc WifiDppTest
Change-Id: I58f61929e4b8792d43fc495ddaa51f766737b743
2 files changed