Undo symlinked libc++fs

Several features for auto need libc++fs, but the previous solution of
symlinking to external/libcxx would cause problems later down the
line. Instead, we will compile a separate version of libc++fs with a
different namespace, as to not interfere with the canonical
std::filesystem.

Change summary
===============================================================
directory_iterator.cpp:
-----------------------
* _LIBCPP_BEGIN_NAMESPACE_FILESYSTEM --> namespace android::hardware::automotive::filesystem {
* _LIBCPP_END_NAMESPACE_FILESYSTEM --> }  // namespace android::hardware::automotive::filesystem
* disable clang-format (to reduce diff size)
* "" --> <> for non-local includes

filesystem_common.h:
--------------------
* FILESYSTEM_COMMON_H --> AUTO_FILESYSTEM_COMMON_H
* "" --> <> for non-local includes
* filesystem --> automotive/filesystem
* don't include apple_availability.h
* add using std::error_code
* _LIBCPP_BEGIN_NAMESPACE_FILESYSTEM --> namespace android::hardware::automotive::filesystem {
* use namespace std::chrono
* use std::error_code, std::is_floating_point, std::micro, std::nano, std::ratio
* LIBCPP_END_NAMESPACE_FILESYSTEM --> }  // namespace android::hardware::automotive::filesystem
* disable clang format (to reduce diff size)

filesystem:
-----------
* _LIBCPP_FILESYSTEM --> _LIBAUTO_FILESYSTEM
* std::filesystem --> android::hardware:automotive::filesystem
* _VSTD_FS --> android::hardware::automotive::filesystem
* _LIBCPP_END_NAMESPACE_FILESYSTEM --> }  // namespace android::hardware::automotive::filesystem
* Copied _FilesystemClock from chrono
* use namespace std and std::chrono
* use std::basic_string, std::enable_if, std::error_code, and std::false_type

operations.cpp:
---------------
* filesystem --> automotive/filesystem
* "" --> <> for non-local includes
* _LIBCPP_BEGIN_NAMESPACE_FILESYSTEM --> namespace android::hardware::automotive::filesystem {
* _VSTD_FS --> android::hardware::automotive::filesystem
* _LIBCPP_END_NAMESPACE_FILESYSTEM --> }  // namespace android::hardware::automotive::filesystem
* disable clang-format (to reduce diff size)
* fix "the the" typo (to pass lint)

Bug: 152067309
Test: Manual
Change-Id: I5551b3e634b85b4d7236e888de68740bfda6aad1