Add DisableConfigSyncTargetPreparer to CTS/MTS

The target preparer ensures that the device does not have change of
DeviceConfig while it is running the tests on S+ (R- is not supported by
the preparer).

This fixes flakes where settings get changed mid-test.

Bug: 210377950
Test: atest CtsNetTestCases
Change-Id: Icfbb402f31b6ee1321e9fcdab994233fb1981bb0
1 file changed