print_tools: Use const methods from libipp in JSON generator This patch removes a temporary workaround originated in lack of const methods in libipp library. It also fixes two issues in the libipp. BUG=chromium:994893 TEST=on nautilus Change-Id: Ic499b160401ab4eb42d9ebe952ab87802578fabe Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform2/+/1949189 Reviewed-by: David Valleau <valleau@chromium.org> Reviewed-by: Sean Kau <skau@chromium.org> Commit-Queue: Piotr Pawliczek <pawliczek@chromium.org> Tested-by: Piotr Pawliczek <pawliczek@chromium.org> Cr-Mirrored-From: https://chromium.googlesource.com/chromiumos/platform2 Cr-Mirrored-Commit: 70ef9a32b025dd78655d22bf6f309b153de14f4a