Don't enforce visiblity on platformBootclasspathImplLibDepTag

When the dependencies that use platformBootclasspathImplLibDepTag
are moved earlier they are checked by the visibility enforcement
mutator.  Implement ExcludeFromVisibilityEnforcementTag so they
don't require visibility.

Test: go test ./...
Change-Id: I6019228561530bd406936fd0d2a16eff471c148d
diff --git a/java/platform_bootclasspath.go b/java/platform_bootclasspath.go
index 152eb1e..86062d4 100644
--- a/java/platform_bootclasspath.go
+++ b/java/platform_bootclasspath.go
@@ -15,6 +15,8 @@
 package java
 
 import (
+	"github.com/google/blueprint"
+
 	"android/soong/android"
 	"android/soong/dexpreopt"
 )
@@ -33,9 +35,18 @@
 	platformBootclasspathArtBootJarDepTag  = bootclasspathDependencyTag{name: "art-boot-jar"}
 	platformBootclasspathBootJarDepTag     = bootclasspathDependencyTag{name: "platform-boot-jar"}
 	platformBootclasspathApexBootJarDepTag = bootclasspathDependencyTag{name: "apex-boot-jar"}
-	platformBootclasspathImplLibDepTag     = dependencyTag{name: "impl-lib-tag"}
 )
 
+type platformBootclasspathImplLibDepTagType struct {
+	blueprint.BaseDependencyTag
+}
+
+func (p platformBootclasspathImplLibDepTagType) ExcludeFromVisibilityEnforcement() {}
+
+var platformBootclasspathImplLibDepTag platformBootclasspathImplLibDepTagType
+
+var _ android.ExcludeFromVisibilityEnforcementTag = platformBootclasspathImplLibDepTag
+
 type platformBootclasspathModule struct {
 	android.ModuleBase
 	ClasspathFragmentBase