Add DisableConfigSyncTargetPreparer to NetworkStackRootTests

DisableConfigSyncTargetPreparer ensures that DeviceConfig sync is
disabled during the test, so the feature flags used in the Root
test will not change. This fixes the flaky tests.

Bug: 257458082
Bug: 255857003
Test: atest NetworkStackRootTests
Change-Id: Ie52ea26587cf0ff76efaf111f6caa601e39b9599
2 files changed