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
diff --git a/tests/cts/net/AndroidTestTemplate.xml b/tests/cts/net/AndroidTestTemplate.xml
index d605799..48a1c79 100644
--- a/tests/cts/net/AndroidTestTemplate.xml
+++ b/tests/cts/net/AndroidTestTemplate.xml
@@ -30,6 +30,8 @@
     </target_preparer>
     <target_preparer class="com.android.testutils.ConnectivityCheckTargetPreparer">
     </target_preparer>
+    <target_preparer class="com.android.testutils.DisableConfigSyncTargetPreparer">
+    </target_preparer>
     <test class="com.android.tradefed.testtype.AndroidJUnitTest" >
         <option name="package" value="android.net.cts" />
         <option name="runtime-hint" value="9m4s" />