Treat core_platform as stable unless module uses legacy

The sdk_version: "core_platform" refers to the stable core platform
unless the module is in the exception list. This change makes sure that
CheckStableSdkVersion() reflects that behavior.

Bug: 180399951
Test: m nothing
Change-Id: Ia0b1e13322352b87f5a3c6621e37f23ba637ffb6