adb: support forwarding TCP port 0.

This CL adds support to forward or reverse TCP port 0 to allow the
system to automatically select an open port. The resolved port number
will be printed to stdout:
  $ adb forward tcp:0 tcp:8000
  12345
  $ adb reverse tcp:0 tcp:9000
  23456
This allows testing to be more robust by not hardcoding TCP ports which
may already be in use.

Forwarding port 0 is a host-only change and will work with any device,
but reversing port 0 requires the device to be updated with a new adbd
binary.

This CL also does a little bit of cleanup such as moving the alistener
class out of adb.h, and adds some error checking and additional tests.

Bug: 28051746
Test: python -m unittest discover
Test: adb_test
Test: `adb forward` and `adb reverse` with tcp:0
Change-Id: Icaa87346685b403ab5da7f0e6aa186aa091da572
diff --git a/adb/adb.cpp b/adb/adb.cpp
index 49d2936..11e9c68 100644
--- a/adb/adb.cpp
+++ b/adb/adb.cpp
@@ -955,18 +955,25 @@
 
         std::string error;
         InstallStatus r;
+        int resolved_tcp_port = 0;
         if (kill_forward) {
             r = remove_listener(pieces[0].c_str(), transport);
         } else {
-            r = install_listener(pieces[0], pieces[1].c_str(), transport,
-                                 no_rebind, &error);
+            r = install_listener(pieces[0], pieces[1].c_str(), transport, no_rebind,
+                                 &resolved_tcp_port, &error);
         }
         if (r == INSTALL_STATUS_OK) {
 #if ADB_HOST
-            /* On the host: 1st OKAY is connect, 2nd OKAY is status */
+            // On the host: 1st OKAY is connect, 2nd OKAY is status.
             SendOkay(reply_fd);
 #endif
             SendOkay(reply_fd);
+
+            // If a TCP port was resolved, send the actual port number back.
+            if (resolved_tcp_port != 0) {
+                SendProtocolString(reply_fd, android::base::StringPrintf("%d", resolved_tcp_port));
+            }
+
             return 1;
         }