PM: Various fixes to Random Provider

* Remove mention of uint64; use uint64_t instead.

* Simpler reading of bytes from /dev/urandom into a uint64_t.

* Replace string concatenation with StringPrintf.

BUG=None
TEST=Builds and passes unit tests.

Change-Id: Ic0760ae4c2b21e4ffc823a676a9439e867bf9f64
Reviewed-on: https://chromium-review.googlesource.com/184326
Reviewed-by: Gilad Arnold <garnold@chromium.org>
Commit-Queue: Gilad Arnold <garnold@chromium.org>
Tested-by: Gilad Arnold <garnold@chromium.org>
diff --git a/policy_manager/random_vars.cc b/policy_manager/random_vars.cc
index 1eb0c0a..95e4eea 100644
--- a/policy_manager/random_vars.cc
+++ b/policy_manager/random_vars.cc
@@ -6,6 +6,6 @@
 
 namespace chromeos_policy_manager {
 
-Variable<uint64>* var_random_seed = NULL;
+Variable<uint64_t>* var_random_seed = NULL;
 
 }  // namespace chromeos_policy_manager