init: Enable ANDROID_BASE_UNIQUE_FD_DISABLE_IMPLICIT_CONVERSION

From the unique_fd.h header file: "unique_fd's operator int is
dangerous, but we have way too much code that depends on it, so make
this opt-in at first."

From the Google C++ style guide: "Do not define implicit conversions."
See also go/cstyle#Implicit_Conversions.

Hence this CL that disables unique_fd::operator int().

Change-Id: I28d94755d5408f63e5819da8d1cbc285057f867f
Signed-off-by: Bart Van Assche <bvanassche@google.com>
diff --git a/init/subcontext.cpp b/init/subcontext.cpp
index 961e006..6a095fb 100644
--- a/init/subcontext.cpp
+++ b/init/subcontext.cpp
@@ -207,7 +207,7 @@
 
         // We explicitly do not use O_CLOEXEC here, such that we can reference this FD by number
         // in the subcontext process after we exec.
-        int child_fd = dup(subcontext_socket);  // NOLINT(android-cloexec-dup)
+        int child_fd = dup(subcontext_socket.get());  // NOLINT(android-cloexec-dup)
         if (child_fd < 0) {
             PLOG(FATAL) << "Could not dup child_fd";
         }
@@ -268,12 +268,12 @@
 }
 
 Result<SubcontextReply> Subcontext::TransmitMessage(const SubcontextCommand& subcontext_command) {
-    if (auto result = SendMessage(socket_, subcontext_command); !result.ok()) {
+    if (auto result = SendMessage(socket_.get(), subcontext_command); !result.ok()) {
         Restart();
         return ErrnoError() << "Failed to send message to subcontext";
     }
 
-    auto subcontext_message = ReadMessage(socket_);
+    auto subcontext_message = ReadMessage(socket_.get());
     if (!subcontext_message.ok()) {
         Restart();
         return Error() << "Failed to receive result from subcontext: " << subcontext_message.error();