libcutils/fastboot: improve multi-buffer write.

Fixes libcutils multi-buffer write interface to be more friendly and
hooks into it from the fastboot Socket class.

Bug: http://b/26558551
Change-Id: Ibb3a8428fc379755602de52722c1260f9e345bc0
diff --git a/fastboot/socket_mock.h b/fastboot/socket_mock.h
index 3e62b33..c48aa7b 100644
--- a/fastboot/socket_mock.h
+++ b/fastboot/socket_mock.h
@@ -56,7 +56,8 @@
     SocketMock();
     ~SocketMock() override;
 
-    ssize_t Send(const void* data, size_t length) override;
+    bool Send(const void* data, size_t length) override;
+    bool Send(std::vector<cutils_socket_buffer_t> buffers) override;
     ssize_t Receive(void* data, size_t length, int timeout_ms) override;
     int Close() override;
     virtual std::unique_ptr<Socket> Accept();
@@ -64,9 +65,6 @@
     // Adds an expectation for Send().
     void ExpectSend(std::string message);
 
-    // Adds an expectation for Send() that returns -1.
-    void ExpectSendFailure(std::string message);
-
     // Adds data to provide for Receive().
     void AddReceive(std::string message);