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/bazel.go b/android/bazel.go
index 1646883..58d9d87 100644
--- a/android/bazel.go
+++ b/android/bazel.go
@@ -32,6 +32,22 @@
 	Bp2BuildTopLevel = "."
 )
 
+type MixedBuildEnabledStatus int
+
+const (
+	// This module can be mixed_built.
+	MixedBuildEnabled = iota
+
+	// There is a technical incompatibility preventing this module from being
+	// bazel-analyzed. Note: the module might also be incompatible.
+	TechnicalIncompatibility
+
+	// This module cannot be mixed_built due to some incompatibility with it
+	// that is not a platform incompatibility. Example: the module-type is not
+	// enabled, or is not bp2build-converted.
+	ModuleIncompatibility
+)
+
 // FileGroupAsLibrary describes a filegroup module that is converted to some library
 // such as aidl_library or proto_library.
 type FileGroupAsLibrary interface {
@@ -346,24 +362,31 @@
 	}).(Bp2BuildConversionAllowlist)
 }
 
-// MixedBuildsEnabled returns true if a module is ready to be replaced by a
-// converted or handcrafted Bazel target. As a side effect, calling this
-// method will also log whether this module is mixed build enabled for
-// metrics reporting.
-func MixedBuildsEnabled(ctx BaseModuleContext) bool {
+// MixedBuildsEnabled returns a MixedBuildEnabledStatus regarding whether
+// a module is ready to be replaced by a converted or handcrafted Bazel target.
+// As a side effect, calling this method will also log whether this module is
+// mixed build enabled for metrics reporting.
+func MixedBuildsEnabled(ctx BaseModuleContext) MixedBuildEnabledStatus {
 	module := ctx.Module()
 	apexInfo := ctx.Provider(ApexInfoProvider).(ApexInfo)
 	withinApex := !apexInfo.IsForPlatform()
+
+	platformIncompatible := isPlatformIncompatible(ctx.Os(), ctx.Arch().ArchType)
+	if platformIncompatible {
+		ctx.Config().LogMixedBuild(ctx, false)
+		return TechnicalIncompatibility
+	}
+
 	mixedBuildEnabled := ctx.Config().IsMixedBuildsEnabled() &&
-		ctx.Os() != Windows && // Windows toolchains are not currently supported.
-		ctx.Os() != LinuxBionic && // Linux Bionic toolchains are not currently supported.
-		ctx.Os() != LinuxMusl && // Linux musl toolchains are not currently supported (b/259266326).
-		ctx.Arch().ArchType != Riscv64 && // TODO(b/262192655) Riscv64 toolchains are not currently supported.
 		module.Enabled() &&
 		convertedToBazel(ctx, module) &&
 		ctx.Config().BazelContext.IsModuleNameAllowed(module.Name(), withinApex)
 	ctx.Config().LogMixedBuild(ctx, mixedBuildEnabled)
-	return mixedBuildEnabled
+
+	if mixedBuildEnabled {
+		return MixedBuildEnabled
+	}
+	return ModuleIncompatibility
 }
 
 // ConvertedToBazel returns whether this module has been converted (with bp2build or manually) to Bazel.
@@ -388,6 +411,13 @@
 	OtherModuleDir(m blueprint.Module) string
 }
 
+func isPlatformIncompatible(osType OsType, arch ArchType) bool {
+	return osType == Windows || // Windows toolchains are not currently supported.
+		osType == LinuxBionic || // Linux Bionic toolchains are not currently supported.
+		osType == LinuxMusl || // Linux musl toolchains are not currently supported (b/259266326).
+		arch == Riscv64 // TODO(b/262192655) Riscv64 toolchains are not currently supported.
+}
+
 func (b *BazelModuleBase) shouldConvertWithBp2build(ctx bazelOtherModuleContext, module blueprint.Module) bool {
 	if !b.bazelProps().Bazel_module.CanConvertToBazel {
 		return false