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

android/neverallow.go

@@ -59,6 +59,7 @@ func init() {
 	AddNeverAllowRules(createInitFirstStageRules()...)
 	AddNeverAllowRules(createProhibitFrameworkAccessRules()...)
 	AddNeverAllowRules(createBp2BuildRule())
+	AddNeverAllowRules(createCcStubsRule())
 }
 
 // Add a NeverAllow rule to the set of rules to apply.
@@ -214,6 +215,17 @@ func createCcSdkVariantRules() []Rule {
 	}
 }
 
+func createCcStubsRule() Rule {
+	ccStubsImplementationInstallableProjectsAllowedList := []string{
+		"packages/modules/Virtualization/vm_payload",
+	}
+
+	return NeverAllow().
+		NotIn(ccStubsImplementationInstallableProjectsAllowedList...).
+		WithMatcher("stubs.implementation_installable", isSetMatcherInstance).
+		Because("implementation_installable can only be used in allowed projects.")
+}
+
 func createUncompressDexRules() []Rule {
 	return []Rule{
 		NeverAllow().