adb: improve names for transport threads.

The old names seems confusing. output_thread was reading remote data and writing to
local sockets. input_thread was reading local sockets data and writing to remote.
This change tries to make it clear by renaming output_thread to read_transport thread,
and renaming input_thread to write_transport thread.

Change-Id: I2e7b4cde7a94d436f3745e9e3ab10780e7caa8ac
diff --git a/usb_linux.cpp b/usb_linux.cpp
index 6ccc8e2..65b8735 100644
--- a/usb_linux.cpp
+++ b/usb_linux.cpp
@@ -592,6 +592,6 @@
     sigaction(SIGALRM, &actions, nullptr);
 
     if (!adb_thread_create(device_poll_thread, nullptr)) {
-        fatal_errno("cannot create input thread");
+        fatal_errno("cannot create device_poll thread");
     }
 }