Fix bp2build's stub/impl selection logic for platform variants

For a dependency edge A --> B (stublib), Soong will link A's platform
variant against impl of B's platform variant if either of these are true
1. A and B have same apex_available
2. B has bootstrap: true
3. B is only available to platform

(3) was missing from bp2build. This CL adds that.

To implement this, we check the `apex_available` property of B.

Test: updated bp2build tests
Test: Built the internal module b/299191635 that was failing due to this
Bug: 299191635
Change-Id: Iafb173a3ab20d69b89f7949ce40c6f4096396f24