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
Bug: 147527408
Change-Id: Ie144891f1c413a18b705f560d70d23041d36772b
Merged-In: Ie144891f1c413a18b705f560d70d23041d36772b