bp2build: add bazel_module: { bp2build_available } prop.

This CL adds a per-target allowlist to instruct bp2build on which modules it should generate Bazel targets for.

Test: soong tests
Change-Id: I869e66fce405c2c6689b381569b8cc0118cbcf76
diff --git a/genrule/genrule.go b/genrule/genrule.go
index 427c995..a500c27 100644
--- a/genrule/genrule.go
+++ b/genrule/genrule.go
@@ -800,7 +800,7 @@
 
 func GenruleBp2Build(ctx android.TopDownMutatorContext) {
 	m, ok := ctx.Module().(*Module)
-	if !ok {
+	if !ok || !m.properties.Bazel_module.Bp2build_available {
 		return
 	}