Use std::filesystem when libc++ is new enough

_LIBCPP_VERSION in external/libcxx is 8000. When _LIBCPP_VERSION is
newer, assume it's the new libc++ toolchain prebuilt, which has a
finalized std::filesystem, and use std::filesystem instead. To make
Soong happy, keep the android.hardware.automotive@libc++fs library for
now but stub out the C++ source files and make the
android::hardware::automotive::filesystem namespace an alias for
std::filesystem.

Bug: 152067309
Bug: 175635923
Test: treehugger
Test: m android.hardware.automotive.can@1.0-service
Test: m android.hardware.automotive@libc++fs
Change-Id: I7aede74cda0122434d972a075d7c7a9933845450