Export libarect_headers to llndk api surface

Previously, we were exporting the transitive headers of libnativewindow
(libarect_headers) only to core and mainline api domains. This CL
explicitly exports these headers to vendor/product via
lldnk.exoprt_llndk_headers

Also, move the headers of libarect from `export_static_lib_headers` to
`export_header_lib_headers` for consistency.

Bug: 254136046

Test: TH
Test: Manual (detailed below)
$ cat << EOF > mytest.cpp
 #include <android/hardware_buffer.h>
EOF

(Add two new Soong modules)
cc_library {
  name: "mytest",
  srcs: ["mytest.cpp"],
  shared_libs: ["libnativewindow"],
}

cc_library {
  name: "mytest_vendor",
  srcs: ["mytest.cpp"],
  shared_libs: ["libnativewindow"],
  vendor: true
}

mytest would build ok before and after change
mytest_vendor builds ok only after change

Change-Id: If9d677545ac29f232c6b296312255a63d5ce850c