Fix: stubs lib is installed in APEX

This CL fixes the problem that when a lib is defined with stubs, the
stubs variantof the lib is installed to the APEX. This was happening
because the non-stubs variant is the last variant of the 'version'
variants and addFarVariationDependencies selects the first variant when
the 'version' variant isn't specified.

Fixing the problem by making the non-stubs variant (whose name is "")
the first variant.

Test: m (apex_test)

Change-Id: I1505fd2f29a0d70c916bad51000aa06f2b80b137