Replace #apex with #systemapi

Currently, for an API symbol to be accessible to APEXes, it needs to be
marked as either # systemapi or # apex. It was originally just # apex,
but we added # systemapi to clearly identify the origin of the APIs.

The intended use is

* #apex is for APEX-visible symbols that are defined in an APEX
* #systemapi is for APEX-visible symbols that are defined in the
  platform (the non-updatable part)

This intention is documented build/soong/docs/map_files.md, but isn't
enforced at all.

With b/239274367, this is now enforced and therefore the #apex tags in
the platform library are replaced with `#systemapi`

This change does not alter any functionality.

Bug: 239274367
Test: m
Change-Id: Ibdb9122c9969749e055404078bc2280edaa7346d

libsync/libsync.map.txt

@@ -19,7 +19,7 @@ LIBSYNC {
     sync_merge; # introduced=26
     sync_file_info; # introduced=26
     sync_file_info_free; # introduced=26
-    sync_wait; # llndk apex
+    sync_wait; # llndk systemapi
     sync_fence_info; # llndk
     sync_pt_info; # llndk
     sync_fence_info_free; # llndk

libvndksupport/libvndksupport.map.txt

@@ -1,8 +1,8 @@
 LIBVNDKSUPPORT {
   global:
-    android_is_in_vendor_process; # llndk apex
-    android_load_sphal_library; # llndk apex
-    android_unload_sphal_library; # llndk apex
+    android_is_in_vendor_process; # llndk systemapi
+    android_load_sphal_library; # llndk systemapi
+    android_unload_sphal_library; # llndk systemapi
   local:
     *;
 };