Fix possible leaks in CanvasContext HintSessionWrapper

Ensure the mHintSessionFuture is accounted for correctly in the
destructor, and proper destruction is taken in CanvasContext::destroy()
in addition to the destructor.

Bug: b/297230599
Test: manual
Change-Id: Idcbd0f2ba87290f090d693f0a64cb46fe8270b95
diff --git a/libs/hwui/renderthread/HintSessionWrapper.h b/libs/hwui/renderthread/HintSessionWrapper.h
index 24b8150..bdb9959 100644
--- a/libs/hwui/renderthread/HintSessionWrapper.h
+++ b/libs/hwui/renderthread/HintSessionWrapper.h
@@ -37,6 +37,7 @@
     void sendLoadResetHint();
     void sendLoadIncreaseHint();
     bool init();
+    void destroy();
 
 private:
     APerformanceHintSession* mHintSession = nullptr;