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
diff --git a/bp2build/cc_library_conversion_test.go b/bp2build/cc_library_conversion_test.go
index 246f169..9bb1713 100644
--- a/bp2build/cc_library_conversion_test.go
+++ b/bp2build/cc_library_conversion_test.go
@@ -2867,7 +2867,6 @@
 		ExpectedBazelTargets: makeCcLibraryTargets("foolib", AttrNameToString{
 			"implementation_dynamic_deps": `select({
         "//build/bazel/rules/apex:foo": ["@api_surfaces//module-libapi/current:barlib"],
-        "//build/bazel/rules/apex:system": ["@api_surfaces//module-libapi/current:barlib"],
         "//conditions:default": [":barlib"],
     })`,
 			"local_includes": `["."]`,
@@ -2925,10 +2924,6 @@
             "@api_surfaces//module-libapi/current:barlib",
             "@api_surfaces//module-libapi/current:quxlib",
         ],
-        "//build/bazel/rules/apex:system": [
-            "@api_surfaces//module-libapi/current:barlib",
-            "@api_surfaces//module-libapi/current:quxlib",
-        ],
         "//conditions:default": [
             ":barlib",
             ":quxlib",
diff --git a/bp2build/cc_library_shared_conversion_test.go b/bp2build/cc_library_shared_conversion_test.go
index 90b13b0..44b9722 100644
--- a/bp2build/cc_library_shared_conversion_test.go
+++ b/bp2build/cc_library_shared_conversion_test.go
@@ -661,7 +661,7 @@
             ":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": [
diff --git a/cc/bp2build.go b/cc/bp2build.go
index 83553c8..62416f7 100644
--- a/cc/bp2build.go
+++ b/cc/bp2build.go
@@ -1624,6 +1624,11 @@
 				if linkable, ok := ctx.Module().(LinkableInterface); ok && linkable.Bootstrap() {
 					sameApiDomain = true
 				}
+				// If dependency has `apex_available: ["//apex_available:platform]`, then the platform variant of rdep should link against its impl.
+				// https://cs.android.com/android/_/android/platform/build/soong/+/main:cc/cc.go;l=3617;bpv=1;bpt=0;drc=c6a93d853b37ec90786e745b8d282145e6d3b589
+				if depApexAvailable := dep.(*Module).ApexAvailable(); len(depApexAvailable) == 1 && depApexAvailable[0] == android.AvailableToPlatform {
+					sameApiDomain = true
+				}
 			} else {
 				sameApiDomain = android.InList(apiDomain, dep.(*Module).ApexAvailable())
 			}