VNDK libs use "unique" APEX variants

In case there are two vendor apexes(one with "use_vndk_as_stable:true",
and the other with "use_vndk_as_stable:false") a VNDK lib used by both
will have "APEX" variant and the former APEX will use "apex" variation.

For example,

 apex1(use_vndk_as_stable) -> foo -> libvndk
 apex2 -> bar -> libvndk

Since foo, bar and libvndk are mutated into two APEX variations("",
"apex10000"), foo will use the apex variation of libvndk.

To fix this, VNDK libs can use "unique" APEX variations. Then, in the
above example, foo will have "myapex1" variation and libvndk will have
two APEX variations("" and "apex2"). So foo will link to ""(non-APEX)
variation as fallback.

Bug: 216847402
Test: m nothing (soong tests)
Change-Id: I116932860ef79e22dc338a58b251e3ca693ab4f3