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

apex/apex.go

@@ -2301,7 +2301,7 @@ func (a *apexBundle) depVisitor(vctx *visitorContext, ctx android.ModuleContext,
 				//
 				// Always include if we are a host-apex however since those won't have any
 				// system libraries.
-				if !am.DirectlyInAnyApex() {
+				if ch.IsStubsImplementationRequired() && !am.DirectlyInAnyApex() {
 					// we need a module name for Make
 					name := ch.ImplementationModuleNameForMake(ctx) + ch.Properties.SubName
 					if !android.InList(name, a.requiredDeps) {