Prebuilt stub not available to platform is handled correctly

When a shared library providing stubs is included in an APEX, only the
stub variant (i.e. version:"1", etc.) gets emitted to Android.mk. This
enforces that everything in the Make world to link to the stub providing
only the public APIs of the library. The non-stub variant (i.e.
version:"") isn't exposed, otherwise, others will be able to access
private part of the lib which isn't guaranteed to be stable.

This has been done by unhiding the stub variant when it is known that
the library is actually included in an APEX. Note that stub variants are
by default hidden.

The above mechanism however doesn't work when the shared library is
replaced by a prebuilt and when the APEX is also replaced by a prebuilt.
Then, the fact that the prebuilt library is actually in the APEX gets
lost.  In that case, AnyVariantDirectlyInAnyApex() returns false for the
prebuilt library. As a result, the stub variant remains hidden and not
emitted to Android.mk.

This change fixes the problem by checking if the lib isn't available for
the platform at all. If not available for the platform (e.g.
apex_available doesn't have "//apex_available:platform"), the lib is
assumed to be included in an APEX even when it actually didn't go
through the apex mutator (... because it's a prebuilt).

Bug: 175166063
Test: m nothing

Change-Id: I41fabd5b368baecf4dc3c5a080b466f8bcd79d77