Use the global default cpp_std when building HWUI

Per b/344590580 and enh@, there can be weird issues with different
shared libraries being built with different C++ versions.

Unless HWUI has a need for gnu++2b (C++23), we may as well fall back
to the global default, which was rev'd to gnu++20 in
aosp/I3637444d74868b3a29b425fd749ecbbe1a45fbf7

Bug: b/344590580
Test: presubmit
Flag: EXEMPT unflaggable build change
Change-Id: Ief8cbb9f50a562909634a92819f43e9a3ae36c7e