SurfaceFlinger: add RootTargetPreparer to SurfaceFlinger_test

Some of SurfaceFlinger tests such as SurfaceInterceptorTest requires
root access as they use sf backdoor interface.

Test: adb unroot; atest SurfaceFlinger_test
Bug: 151830811
Change-Id: Ie144891f1c413a18b705f560d70d23041d36772b
diff --git a/services/surfaceflinger/tests/AndroidTest.xml b/services/surfaceflinger/tests/AndroidTest.xml
index 8315037..000628f 100644
--- a/services/surfaceflinger/tests/AndroidTest.xml
+++ b/services/surfaceflinger/tests/AndroidTest.xml
@@ -18,6 +18,7 @@
         <option name="cleanup" value="true" />
         <option name="push" value="SurfaceFlinger_test->/data/local/tmp/SurfaceFlinger_test" />
     </target_preparer>
+    <target_preparer class="com.android.tradefed.targetprep.RootTargetPreparer"/>
     <option name="test-suite-tag" value="apct" />
     <test class="com.android.tradefed.testtype.GTest" >
         <option name="native-test-device-path" value="/data/local/tmp" />