Add apex compat symlinks to LOCAL_SOONG_INSTALL_SYMLINKS

We want to make FULL_SYSTEMIMAGE_DEPS more accurate so that we can
remove the need for `m installclean`.

Currently, the compat symlinks are installed by adding them as
dependencies of other installed files. Doing this doesn't end up
propagaing the compat symlinks all the way to FULL_SYSTEMIMAGE_DEPS.

Add them to LOCAL_SOONG_INSTALL_SYMLINKS, which will cause them to
be propagated to FULL_SYSTEMIMAGE_DEPS.

Bug: 205632228
Test: Full build with a change to delete files not in FULL_SYSTEMIMAGE_DEPS from the staging dir
Change-Id: Idc7fc0a0dbf7c8ac3f0628c16413cd1df69a7821