android:binder: enable preempt before alloc_fd

 __alloc_fd() might sleep so enable preempt before it.

Change-Id: I3328b423e89d6f267724f3efe053ef1fe4040e8b
Signed-off-by: Yi-wei Zhao <gbjc64@motorola.com>
Reviewed-on: http://gerrit.mot.com/857888


SME-Granted: SME Approvals Granted
SLTApproved: Slta Waiver <sltawvr@motorola.com>
Tested-by: Jira Key <jirakey@motorola.com>
Submit-Approved: Jira Key <jirakey@motorola.com>