libbinder: allow stability downgrade

For instance, use a VINTF interface inside the system image without
adding it to the VINTF manifest (e.g. for testing, in this case, the
interface can't be used between system and vendor).

Bug: 183154648
Test: binderStabilityTest
Change-Id: I38019e226547b2437b920e79c252c7e55c17f7e4
diff --git a/libs/binder/tests/binderStabilityTest.cpp b/libs/binder/tests/binderStabilityTest.cpp
index 1f2779a..4270540 100644
--- a/libs/binder/tests/binderStabilityTest.cpp
+++ b/libs/binder/tests/binderStabilityTest.cpp
@@ -131,6 +131,17 @@
     EXPECT_TRUE(Stability::requiresVintfDeclaration(BadStableBinder::vintf()));
 }
 
+TEST(BinderStability, ForceDowngradeStability) {
+    sp<IBinder> someBinder = BadStableBinder::vintf();
+
+    EXPECT_TRUE(Stability::requiresVintfDeclaration(someBinder));
+
+    // silly to do this after already using the binder, but it's for the test
+    Stability::forceDowngradeCompilationUnit(someBinder);
+
+    EXPECT_FALSE(Stability::requiresVintfDeclaration(someBinder));
+}
+
 TEST(BinderStability, VintfStabilityServerMustBeDeclaredInManifest) {
     sp<IBinder> vintfServer = BadStableBinder::vintf();