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

cc/bp2build.go

@@ -1624,6 +1624,11 @@ func setStubsForDynamicDeps(ctx android.BazelConversionPathContext, axis bazel.C
 				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())
 			}