Always set apex_available in SDK snapshots.

Also fix //apex_available:platform that could get lost in the generated
apex_available properties. It only happened if a module didn't specify
an apex_available setting and then got extra entries from
apex.BaselineApexAvailable.

Test: Build ART SDK with libnativeloader_lazy in the SDK, and check
  that the generated prebuilt module has //apex_available:platform
  (before https://r.android.com/1480559).
Change-Id: I9df20aee63455932a7e558993f0f26769cb02792