TTS: Fix binders leak due to a circular dependency

A circular dependency was found which keeps binders alive
even after shutdown:
[client]
SystemConnection -->
SystemConnection#ITextToSpeechSession -->
[server]
TextToSpeechSessionConnection -->
TextToSpeechSessionConnection#ITextToSpeechSessionCallback -->
[client]
SystemConnection -->
...

Anulling SystemConnection#ITextToSpeechSession and
TextToSpeechSessionConnection#ITextToSpeechSessionCallback
on disconnection should make it available for garbage collection
since then no intra-process references would remain.

Fix confirmed with ahat.

Bug: 233615470
Test: atest
Change-Id: I7c9d25b7306d9ce68098c5a6ba0e65711daa3c25

core/java/android/speech/tts/TextToSpeech.java

@@ -2422,6 +2422,7 @@ public class TextToSpeech {
                     @Override
                     public void onDisconnected() {
                         onServiceDisconnected(/* componentName= */ null);
+                        mSession = null;
                     }
 
                     @Override

services/texttospeech/java/com/android/server/texttospeech/TextToSpeechManagerPerUserService.java

@@ -83,7 +83,7 @@ final class TextToSpeechManagerPerUserService extends
             ServiceConnector.Impl<ITextToSpeechService> {
 
         private final String mEngine;
-        private final ITextToSpeechSessionCallback mCallback;
+        private ITextToSpeechSessionCallback mCallback;
         private final DeathRecipient mUnbindOnDeathHandler;
 
         static void start(Context context, @UserIdInt int userId, String engine,
@@ -156,6 +156,7 @@ final class TextToSpeechManagerPerUserService extends
                 } catch (NoSuchElementException ex) {
                     Slog.d(TAG, "The death recipient was not linked.");
                 }
+                mCallback = null;
             }
         }