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

ui/build/soong.go

@@ -288,6 +288,9 @@ func bootstrapBlueprint(ctx Context, config Config) {
 	if config.buildFromTextStub {
 		mainSoongBuildExtraArgs = append(mainSoongBuildExtraArgs, "--build-from-text-stub")
 	}
+	if config.ensureAllowlistIntegrity {
+		mainSoongBuildExtraArgs = append(mainSoongBuildExtraArgs, "--ensure-allowlist-integrity")
+	}
 
 	queryviewDir := filepath.Join(config.SoongOutDir(), "queryview")
 	// The BUILD files will be generated in out/soong/.api_bp2build (no symlinks to src files)