[SurfaceFlinger] Allow root to capture screen.

If adb is running in root, then uid is AID_ROOT and capture screen will fail.
This patch allows root to capture screen.

BUG: 132777517
Test: adb shell screencap -d 0 -p /sdcard/test.png produces a valid screenshot
Change-Id: Ib6eaad2dffa7a68f4b1fce548814f0c2f172c3fd
1 file changed