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

cc/library.go

@@ -71,6 +71,12 @@ type LibraryProperties struct {
 		// List versions to generate stubs libs for. The version name "current" is always
 		// implicitly added.
 		Versions []string
+
+		// Whether to not require the implementation of the library to be installed if a
+		// client of the stubs is installed. Defaults to true; set to false if the
+		// implementation is made available by some other means, e.g. in a Microdroid
+		// virtual machine.
+		Implementation_installable *bool
 	}
 
 	// set the name of the output
@@ -1339,6 +1345,7 @@ type versionedInterface interface {
 	buildStubs() bool
 	setBuildStubs(isLatest bool)
 	hasStubsVariants() bool
+	isStubsImplementationRequired() bool
 	setStubsVersion(string)
 	stubsVersion() string
 
@@ -2298,6 +2305,10 @@ func (library *libraryDecorator) hasStubsVariants() bool {
 		len(library.Properties.Stubs.Versions) > 0
 }
 
+func (library *libraryDecorator) isStubsImplementationRequired() bool {
+	return BoolDefault(library.Properties.Stubs.Implementation_installable, true)
+}
+
 func (library *libraryDecorator) stubsVersions(ctx android.BaseMutatorContext) []string {
 	if !library.hasStubsVariants() {
 		return nil