Do not add _platform suffix to non-apex variation

This commit renames `_platform` suffix to `` (empty string) so that
non-apex variations are not renamed to `_core_shared_platform` or
`_vendor_shared_platform`.

This commit makes sure that `_core_shared` and `_vendor_shared` is
always under `$OUT_DIR/soong` regardless the usages from apex modules.
Furthermore, this avoids the confusing stale lsdump files (e.g. both
`_core_shared` and `_core_shared_platform exist) while creating
reference ABI dumps for VNDK ABI checks.

Bug: 121986692
Test: lunch aosp_arm64-userdebug; make  # no more _platform variants.
Change-Id: Ic02a60ac45f982580349661c22331d114617fd92