Revert "libbinder: disallow APEX at build time"

Build breakage.

This reverts commit 8852cc78a0f0c02caf61f21cb3c520583025ea69.

Bug: 161926892
Reason for revert: b/165698873 - broken build

Change-Id: I4708d5e5aaba655b3399bf59ba342f303af7a8ff
diff --git a/libs/binder/Parcel.cpp b/libs/binder/Parcel.cpp
index 7fb18c5..8fd59ba 100644
--- a/libs/binder/Parcel.cpp
+++ b/libs/binder/Parcel.cpp
@@ -519,13 +519,7 @@
     }
 }
 
-#ifdef __ANDROID_APEX__
-#error APEX cannot include libbinder. The wire protocol for libbinder is not\
-    frozen, and including it in an APEX would also require APEX\
-    servicemanagers which is not an options. Use libbinder_ndk instead.
-#endif
-
-#if defined(__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');