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/android/config.go b/android/config.go
index 032172d..7141e54 100644
--- a/android/config.go
+++ b/android/config.go
@@ -102,6 +102,8 @@
 	UseBazelProxy bool
 
 	BuildFromTextStub bool
+
+	EnsureAllowlistIntegrity bool
 }
 
 // Build modes that soong_build can run as.
@@ -278,6 +280,11 @@
 	// If buildFromTextStub is true then the Java API stubs are
 	// built from the signature text files, not the source Java files.
 	buildFromTextStub bool
+
+	// If ensureAllowlistIntegrity is true, then the presence of any allowlisted
+	// modules that aren't mixed-built for at least one variant will cause a build
+	// failure
+	ensureAllowlistIntegrity bool
 }
 
 type deviceConfig struct {
@@ -1904,6 +1911,10 @@
 	return Bool(c.productVariables.HostMusl)
 }
 
+func (c *config) GetMixedBuildsEnabledModules() map[string]struct{} {
+	return c.mixedBuildEnabledModules
+}
+
 func (c *config) LogMixedBuild(ctx BaseModuleContext, useBazel bool) {
 	moduleName := ctx.Module().Name()
 	c.mixedBuildsLock.Lock()