idmap2: fix cpplint warnings

Fix the following to make cpplint happy again:

  - Use C++ style casts
  - Rename a few header guards
  - Add missing includes of standard C++ headers
  - Add comments to the end of namespaces

Also, for consistency, always use "..." instead of <...> for local
include files, i.e.

  #include "idmap2/.*"

Test: cmds/idmap2/static-checks.sh
Change-Id: Ie735a36d562d1fc51b61f1f0f45aec0d160f602d
diff --git a/cmds/idmap2/libidmap2/PolicyUtils.cpp b/cmds/idmap2/libidmap2/PolicyUtils.cpp
index fc5182a..4e3f54d2 100644
--- a/cmds/idmap2/libidmap2/PolicyUtils.cpp
+++ b/cmds/idmap2/libidmap2/PolicyUtils.cpp
@@ -17,6 +17,8 @@
 #include "include/idmap2/PolicyUtils.h"
 
 #include <sstream>
+#include <string>
+#include <vector>
 
 #include "android-base/strings.h"
 #include "idmap2/Policies.h"