Allow stubs implementation to be omitted

When defining a stubs library, allow specifying that the
implementation library does not need to be installed. This allows for
cases where the implementation is deployed in some non-standard way -
e.g. inside a Microdroid virtual machine.

Without this, we get build errors like: "TARGET module
com.android.compos requires non-existent TARGET module: libvm_payload".

Default behavior is unchanged. The change is protected by an allowlist
to limit usage to the immediate use case.

Bug: 243512108
Test: builds; soong tests pass
Test: Remove allowlist, see build failure
Change-Id: Iaae75f2e93b842f5944a7518cc95069d62c5a638
diff --git a/cc/cc.go b/cc/cc.go
index 306e483..c83ed71 100644
--- a/cc/cc.go
+++ b/cc/cc.go
@@ -1376,6 +1376,13 @@
 	return false
 }
 
+func (c *Module) IsStubsImplementationRequired() bool {
+	if lib := c.library; lib != nil {
+		return lib.isStubsImplementationRequired()
+	}
+	return false
+}
+
 // If this is a stubs library, ImplementationModuleName returns the name of the module that contains
 // the implementation.  If it is an implementation library it returns its own name.
 func (c *Module) ImplementationModuleName(ctx android.BaseModuleContext) string {