A test cc module is installable even if it's not available for platform

This change fixes a bug that test modules (cc_test or its sub types) are
unconditionally considered as non-installable if they are configured to
not available for platform. The rationale behind the decision was that
an APEX variant of a module doesn't need to be installed to the device
because the variant will anyway be included in the APEX. However, it's
wrong for test modules. They are not included in the APEX, but should be
installed to /data/nativetest*.

One might think that we need to make the tests available for the
platform (i.e. apex_available: ["//apex_available:platform"]). This
however doesn't work if the libraries that the tests should link against
are configured to be not available for the platform, which currently is
the case for the ART tests.

Bug: 146995717
Test: m
Change-Id: I51843f5b4ea0a418c64c63784347231590cd3c35