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

cmds/idmap2/Android.bp

@@ -35,8 +35,10 @@ cc_defaults {
     tidy_checks_as_errors: [
         "modernize-*",
         "-modernize-avoid-c-arrays",
+        "-modernize-concat-nested-namespaces",
         "-modernize-pass-by-value",
         "-modernize-replace-disallow-copy-and-assign-macro",
+        "-modernize-return-braced-init-list",
         "-modernize-use-equals-default",
         "-modernize-use-nodiscard",
         "-modernize-use-override",