Add another PowerVR exception for FileSystemPermissionTest.

On platforms with a PowerVR GPU and newer device drivers, the /dev/pvr_sync
device node may be created to support the Android Native Fence Sync
functionality present in JellyBean MR1 and later (if enabled in the platform
and GPU driver).

This device node is used by any process using EGL/GLES, which includes system
processes such as SurfaceFlinger and most sandboxed native/java applications.

It is therefore not possible to use only group-writable permissions.

Imagination Technologies believe that the pvr_sync device driver is secure and
has been properly audited.

Change-Id: I60f56303e6fa015721bb5a034e6d47f57a5e2176