When entering a secure desktop, cancel speech (#14135)

Summary of the issue:
NVDA 2022.2 would cancel speech when handling the foreground event for entering a secure desktop.

The security patch for 2022.2.1 broke how the foreground event was handled, and subsequently #14105 opted to avoid the foreground event entirely.

This results in NVDA not correctly cancelling speech when entering a secure desktop.

Description of user facing changes
Speech is cancelled when entering a secure desktop

Description of development approach
Cancel speech just before enter sleep mode, when SecureDesktopNVDAObject handles it's gain focus event.

source/IAccessibleHandler/__init__.py

@@ -798,7 +798,18 @@ def _get_role(self):
 		return controlTypes.Role.PANE
 
 	def event_gainFocus(self):
-		super(SecureDesktopNVDAObject, self).event_gainFocus()
+		from speech.speech import cancelSpeech
+		# NVDA announces the secure desktop when handling the gainFocus event.
+		# Before announcing the secure desktop and entering sleep mode,
+		# cancel speech so that speech does not overlap with the new instance of NVDA
+		# started on the secure desktop.
+		# Cancelling speech was previously handled by a foreground event,
+		# fired when focusing SecureDesktopNVDAObject.
+		# The foreground event would incorrectly fire on the foreground window of the user desktop.
+		# This foreground event is now explicitly prevented due to the security concerns of
+		# setting the foreground object to an object 'below the lock screen'.
+		cancelSpeech()
+		super().event_gainFocus()
 		# After handling the focus, NVDA should sleep while the secure desktop is active.
 		self.sleepMode = self.SLEEP_FULL