idmap2: fix for compiler update.

Suppress below errors:

include/idmap2/Idmap.h:126:12: error: avoid repeating the return type
from the declaration; use a braced initializer list instead
[modernize-return-braced-init-list,-warnings-as-errors]
    return StringPiece(target_path_);
           ^~~~~~~~~~~~            ~
           {                       }

gen/aidl/android/os/BnIdmap2.h:6:1: error: nested namespaces can be concatenated
[modernize-concat-nested-namespaces,-warnings-as-errors]
namespace android {
^~~~~~~~~~~~~~~~~~~
namespace android::os

Bug: 193712493
Test: build with the next compiler
Change-Id: I14297355dd92c2bbf4bd24da95cfa868b427739e