libbinder: Consider 'VNDK + APEX' as system stable

Currently, we have 'use_vendor' on APEX which means that
'__ANDROID_VNDK__' things might not be logically coupled with the vendor
partition. Since APEXes can't be used from the vendor partition now, we
can mark 'vndk && apex' things as having a stability corresponding to
the system partition.

Bug: 136027762
Test: boot && binderStabilityTest
Change-Id: Ib784c0da6bbb4435aec0f8c1fd1bf421332d64d7
diff --git a/libs/binder/Parcel.cpp b/libs/binder/Parcel.cpp
index ee26997..1131732 100644
--- a/libs/binder/Parcel.cpp
+++ b/libs/binder/Parcel.cpp
@@ -509,7 +509,7 @@
     }
 }
 
-#ifdef __ANDROID_VNDK__
+#if defined(__ANDROID_VNDK__) && !defined(__ANDROID_APEX__)
 constexpr int32_t kHeader = B_PACK_CHARS('V', 'N', 'D', 'R');
 #else
 constexpr int32_t kHeader = B_PACK_CHARS('S', 'Y', 'S', 'T');