Remove gBn/sConstructorMap from headers.

Prebuilts that include toBinder() should no longer
link to gBnConstructorMap after rebuild.

After prebuilts are updated, symbols (in Static.cpp and
HidlPassthroughSupport.cpp) can be removed as well.

Test: walleye boots
Test: HAL def libs that link to libhidltransport no longer
      links to gBnConstructorMap / gBsConstructorMap.

Bug: 69122224
Change-Id: I6554a98a3068f9595666b7131df310e200551e45
diff --git a/transport/HidlPassthroughSupport.cpp b/transport/HidlPassthroughSupport.cpp
index e5eb164..c5ca1c9 100644
--- a/transport/HidlPassthroughSupport.cpp
+++ b/transport/HidlPassthroughSupport.cpp
@@ -25,6 +25,9 @@
 namespace hardware {
 namespace details {
 
+// TODO(b/69122224) remove once prebuilts are updated.
+extern BsConstructorMap gBsConstructorMap;
+
 static sp<IBase> tryWrap(const std::string& descriptor, sp<IBase> iface) {
     auto func = getBsConstructorMap().get(descriptor, nullptr);
     if (!func) {