Set filecontext for the test apex

Label the surfaceflinger binary as surfaceflinger_exec

Test: m apex.test; m; device is bootable

Change-Id: I22297ba514f7ba298a1d0d4b476a447f26e48078

apex/apex.test-file_contexts

 /manifest\.json   u:object_r:system_file:s0
 (/.*)?            u:object_r:system_file:s0
+/bin/surfaceflinger u:object_r:surfaceflinger_exec:s0