Use updated serial when running fastboot command (#945)

Make fastboot follow `AndroidDevice.update_serial`

mobly/controllers/android_device_lib/fastboot.py

@@ -60,16 +60,17 @@ class FastbootProxy:
 
   def __init__(self, serial=''):
     self.serial = serial
-    if serial:
-      self.fastboot_str = 'fastboot -s {}'.format(serial)
-    else:
-      self.fastboot_str = 'fastboot'
+
+  def fastboot_str(self):
+    if self.serial:
+      return 'fastboot -s {}'.format(self.serial)
+    return 'fastboot'
 
   def _exec_fastboot_cmd(self, name, arg_str):
-    return exe_cmd(' '.join((self.fastboot_str, name, arg_str)))
+    return exe_cmd(' '.join((self.fastboot_str(), name, arg_str)))
 
   def args(self, *args):
-    return exe_cmd(' '.join((self.fastboot_str,) + args))
+    return exe_cmd(' '.join((self.fastboot_str(),) + args))
 
   def __getattr__(self, name):
     def fastboot_call(*args):

tests/mobly/controllers/android_device_lib/fastboot_test.py

@@ -43,6 +43,63 @@ def test_fastboot_commands_and_results_are_logged_to_debug_log(
         123,
     )
 
+  @mock.patch('mobly.controllers.android_device_lib.fastboot.Popen')
+  def test_fastboot_without_serial(self, mock_popen):
+    expected_stdout = 'stdout'
+    expected_stderr = b'stderr'
+    mock_popen.return_value.communicate = mock.Mock(
+        return_value=(expected_stdout, expected_stderr)
+    )
+    mock_popen.return_value.returncode = 123
+
+    fastboot.FastbootProxy().fake_command('extra', 'flags')
+
+    mock_popen.assert_called_with(
+        'fastboot fake-command extra flags',
+        stdout=mock.ANY,
+        stderr=mock.ANY,
+        shell=True,
+    )
+
+  @mock.patch('mobly.controllers.android_device_lib.fastboot.Popen')
+  def test_fastboot_with_serial(self, mock_popen):
+    expected_stdout = 'stdout'
+    expected_stderr = b'stderr'
+    mock_popen.return_value.communicate = mock.Mock(
+        return_value=(expected_stdout, expected_stderr)
+    )
+    mock_popen.return_value.returncode = 123
+
+    fastboot.FastbootProxy('ABC').fake_command('extra', 'flags')
+
+    mock_popen.assert_called_with(
+        'fastboot -s ABC fake-command extra flags',
+        stdout=mock.ANY,
+        stderr=mock.ANY,
+        shell=True,
+    )
+
+  @mock.patch('mobly.controllers.android_device_lib.fastboot.Popen')
+  def test_fastboot_update_serial(self, mock_popen):
+    expected_stdout = 'stdout'
+    expected_stderr = b'stderr'
+    mock_popen.return_value.communicate = mock.Mock(
+        return_value=(expected_stdout, expected_stderr)
+    )
+    mock_popen.return_value.returncode = 123
+
+    fut = fastboot.FastbootProxy('ABC')
+    fut.fake_command('extra', 'flags')
+    fut.serial = 'XYZ'
+    fut.fake_command('extra', 'flags')
+
+    mock_popen.assert_called_with(
+        'fastboot -s XYZ fake-command extra flags',
+        stdout=mock.ANY,
+        stderr=mock.ANY,
+        shell=True,
+    )
+
 
 if __name__ == '__main__':
   unittest.main()