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_utils.cpp b/adb/adb_utils.cpp
index 3333fc6..5d4755f 100644
--- a/adb/adb_utils.cpp
+++ b/adb/adb_utils.cpp
@@ -27,6 +27,7 @@
 #include <algorithm>
 
 #include <android-base/logging.h>
+#include <android-base/parseint.h>
 #include <android-base/stringprintf.h>
 #include <android-base/strings.h>
 
@@ -230,3 +231,26 @@
     return true;
 }
 #endif
+
+bool forward_targets_are_valid(const std::string& source, const std::string& dest,
+                               std::string* error) {
+    if (android::base::StartsWith(source, "tcp:")) {
+        // The source port may be 0 to allow the system to select an open port.
+        int port;
+        if (!android::base::ParseInt(&source[4], &port) || port < 0) {
+            *error = android::base::StringPrintf("Invalid source port: '%s'", &source[4]);
+            return false;
+        }
+    }
+
+    if (android::base::StartsWith(dest, "tcp:")) {
+        // The destination port must be > 0.
+        int port;
+        if (!android::base::ParseInt(&dest[4], &port) || port <= 0) {
+            *error = android::base::StringPrintf("Invalid destination port: '%s'", &dest[4]);
+            return false;
+        }
+    }
+
+    return true;
+}