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

bp2build/cc_library_shared_conversion_test.go

@@ -661,7 +661,7 @@ cc_library_shared {
             ":libapexfoo_stable",
         ],
         "//build/bazel/rules/apex:system": [
-            "@api_surfaces//module-libapi/current:libplatform_stable",
+            ":libplatform_stable",
             "@api_surfaces//module-libapi/current:libapexfoo_stable",
         ],
         "//conditions:default": [