staging: android: binder.c: Use more appropriate functions for euid retrieval

Instead of getting the reference to whole credential structure, use
task_euid() and current_euid() to get it.

Change-Id: Id5b9d0305b5f90023415863e569988023aaae290
Signed-off-by: Tair Rzayev <tair.rzayev@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>