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/sysdeps_win32.cpp b/adb/sysdeps_win32.cpp
index bc09fdc..faf7f3e 100644
--- a/adb/sysdeps_win32.cpp
+++ b/adb/sysdeps_win32.cpp
@@ -1128,6 +1128,24 @@
     return result;
 }
 
+int adb_socket_get_local_port(int fd) {
+    sockaddr_storage addr_storage;
+    socklen_t addr_len = sizeof(addr_storage);
+
+    if (adb_getsockname(fd, reinterpret_cast<sockaddr*>(&addr_storage), &addr_len) < 0) {
+        D("adb_socket_get_local_port: adb_getsockname failed: %s", strerror(errno));
+        return -1;
+    }
+
+    if (!(addr_storage.ss_family == AF_INET || addr_storage.ss_family == AF_INET6)) {
+        D("adb_socket_get_local_port: unknown address family received: %d", addr_storage.ss_family);
+        errno = ECONNABORTED;
+        return -1;
+    }
+
+    return ntohs(reinterpret_cast<sockaddr_in*>(&addr_storage)->sin_port);
+}
+
 int  adb_shutdown(int  fd)
 {
     FH   f = _fh_from_int(fd, __func__);
@@ -1154,9 +1172,7 @@
     int server = -1;
     int client = -1;
     int accepted = -1;
-    sockaddr_storage addr_storage;
-    socklen_t addr_len = sizeof(addr_storage);
-    sockaddr_in* addr = nullptr;
+    int local_port = -1;
     std::string error;
 
     server = network_loopback_server(0, SOCK_STREAM, &error);
@@ -1165,20 +1181,14 @@
         goto fail;
     }
 
-    if (adb_getsockname(server, reinterpret_cast<sockaddr*>(&addr_storage), &addr_len) < 0) {
-        D("adb_socketpair: adb_getsockname failed: %s", strerror(errno));
+    local_port = adb_socket_get_local_port(server);
+    if (local_port < 0) {
+        D("adb_socketpair: failed to get server port number: %s", error.c_str());
         goto fail;
     }
+    D("adb_socketpair: bound on port %d", local_port);
 
-    if (addr_storage.ss_family != AF_INET) {
-        D("adb_socketpair: unknown address family received: %d", addr_storage.ss_family);
-        errno = ECONNABORTED;
-        goto fail;
-    }
-
-    addr = reinterpret_cast<sockaddr_in*>(&addr_storage);
-    D("adb_socketpair: bound on port %d", ntohs(addr->sin_port));
-    client = network_loopback_client(ntohs(addr->sin_port), SOCK_STREAM, &error);
+    client = network_loopback_client(local_port, SOCK_STREAM, &error);
     if (client < 0) {
         D("adb_socketpair: failed to connect client: %s", error.c_str());
         goto fail;