Mark `utils.get_available_host_port` deprecated (#907)

Getting host port then applying it is inherently unreliable.
Letting the OS handle it is better.
diff --git a/mobly/utils.py b/mobly/utils.py
index 4d5dfc6..02125ed 100644
--- a/mobly/utils.py
+++ b/mobly/utils.py
@@ -596,6 +596,8 @@
 def get_available_host_port():
   """Gets a host port number available for adb forward.
 
+  DEPRECATED: This method is unreliable. Pass `tcp:0` to adb forward instead.
+
   Returns:
     An integer representing a port number on the host available for adb
     forward.
@@ -603,6 +605,11 @@
   Raises:
     Error: when no port is found after MAX_PORT_ALLOCATION_RETRY times.
   """
+  logging.warning(
+      'The method mobly.utils.get_available_host_port is deprecated because it '
+      'is unreliable. Pass "tcp:0" to adb forward instead.'
+  )
+
   # Only import adb module if needed.
   from mobly.controllers.android_device_lib import adb