NotificationManager: clean calling identity in dump().

When a device has work profile, "dumpsys notifiaction" often crashes
if the caller doesn't have the CROSS_USER permission.

The dump() method is protected with checkDumpAndUsageStatsPermission(),
and it won't need further permission check.

Test: "dumpsys notification"
Change-Id: Ief5269dec1e334bcd96013c349a468a0136a9551