Create new target-files-dir target

Create a new phony target target-files-dir, which points to
$(BUILT_TARGET_FILES_DIR) . All dependencies of
BUILT_TARGET_FILES_PACKAGE are now a dependency of
BUILT_TARGET_FILES_DIR. The purpose of BUILT_TARGET_FILES_DIR is to
represent an unzipped version of the target_files.zip . Some tools, such
as ota_from_target_files, can work directly with unzipped target_files.
This change allows such tools to avoid having to zip the target_files
by depending on BUILT_TARGET_FILES_DIR instead of
BUILT_TARGET_FILES_PACKAGE.

This CL does not have any functional changes, targets such as otapackage
and updatepackage still depend on the zipped target_files.

Bug: 227848550
Bug: 277028723

Test: m installclean && m updatepackage
Change-Id: I83e69370ae5efa9046710f5909276077796f6b30