Refactor MixedBuildsEnabled and add --ensure-allowlist-integrity.

Currently, there is little verification around allowlisted modules
actually being mixed-built. This flag would allow us to verify
that a module allowlisted is mixed-built for at least one variant.

Bug: 278910100
Test: m nothing --bazel-mode-staging --ensure-allowlist-integrity
Test: m nothing --bazel-mode-staging --ensure-allowlist-integrity
--bazel-force-enabled-modules=com.google.android.neuralnetworks (This
fails, as expected)
Test: build/soong/test/mixed_mode_test.sh

Change-Id: Icd5976f4f44f1a8caca1e5247d986642f7995f97
diff --git a/cc/cc_test.go b/cc/cc_test.go
index 787669c..fde6bff 100644
--- a/cc/cc_test.go
+++ b/cc/cc_test.go
@@ -59,7 +59,7 @@
 func mixedBuildsPrepareMutator(ctx android.BottomUpMutatorContext) {
 	if m := ctx.Module(); m.Enabled() {
 		if mixedBuildMod, ok := m.(android.MixedBuildBuildable); ok {
-			if mixedBuildMod.IsMixedBuildSupported(ctx) && android.MixedBuildsEnabled(ctx) {
+			if mixedBuildMod.IsMixedBuildSupported(ctx) && android.MixedBuildsEnabled(ctx) == android.MixedBuildEnabled {
 				mixedBuildMod.QueueBazelCall(ctx)
 			}
 		}