ANDROID: binder: init desired_prio.sched_policy before use it

In function binder_transaction_priority(), we access
desired_prio before initialzing it.

This patch fix this.

Change-Id: I9d14d50f9a128010476a65b52631630899a44633
Signed-off-by: Ganesh Mahendran <opensource.ganesh@gmail.com>