Export any platform_compat_config used by apex to sdk snapshot

Previously, the platform_compat_config modules needed to be explicitly
listed in the sdk snapshot and the apex. This change will automatically
export them to the sdk snapshot when the apex is listed in its apexes
property.

Bug: 232401814
Test: m tethering-module-sdk
      # Before this change the generated snapshot did not contain the
      # platform_compat_config, after this change it did.
      m art-module-sdk
      # As that explicitly specifies the platform_compat_config in its
      # compat_configs property this change has no effect.
Change-Id: Ia854b9a52db2b1619fca41a387ce98d7f9f9efe9