Use unique apex variations for bootclasspath fragments

Upcoming changes to the ApexInfo mutator will require unique apex
variations for the bootclasspath fragments, change them now to simplify
comparisons.

Test: go test ./...
Change-Id: Ia8880fe5ac22a7bd2fb9ddaeb3b04b9ae90b1701
diff --git a/java/bootclasspath_fragment.go b/java/bootclasspath_fragment.go
index 8fb8ba9..f6d6cad 100644
--- a/java/bootclasspath_fragment.go
+++ b/java/bootclasspath_fragment.go
@@ -290,6 +290,10 @@
 	return m
 }
 
+func (m *BootclasspathFragmentModule) UniqueApexVariations() bool {
+	return true
+}
+
 func (m *BootclasspathFragmentModule) bootclasspathFragmentPropertyCheck(ctx android.ModuleContext) {
 	contents := m.properties.Contents.GetOrDefault(ctx, nil)
 	if len(contents) == 0 {