[RenderEngine] Reorder when ImageManager thread starts

Previously the background thread for managing EglImages was intialized
as a member variable before the constructor completed running. In very
rare circumstances this can potentially stop the running thread if the
thread ran before mRunning was initialized. To be less error-prone,
require that ImageManger::initThread be explicitly called to guarantee
that class member variables are well-defined when the thread begins
executing.

In case the issue resurfaces after this change, added a debug log to add
context.

Bug: 146416748
Test: builds. There is no reliable repro, so this fix is speculative
Test: systrace to verify thread is running and configured correctly.
Change-Id: I141c86240b90c7f87c22b3768c2e188293987b76
diff --git a/libs/renderengine/gl/ImageManager.h b/libs/renderengine/gl/ImageManager.h
index b5ba554..be67de8 100644
--- a/libs/renderengine/gl/ImageManager.h
+++ b/libs/renderengine/gl/ImageManager.h
@@ -39,6 +39,10 @@
     };
     ImageManager(GLESRenderEngine* engine);
     ~ImageManager();
+    // Starts the background thread for the ImageManager
+    // We need this to guarantee that the class is fully-constructed before the
+    // thread begins running.
+    void initThread();
     void cacheAsync(const sp<GraphicBuffer>& buffer, const std::shared_ptr<Barrier>& barrier)
             EXCLUDES(mMutex);
     status_t cache(const sp<GraphicBuffer>& buffer);
@@ -57,7 +61,7 @@
     void queueOperation(const QueueEntry&& entry);
     void threadMain();
     GLESRenderEngine* const mEngine;
-    std::thread mThread = std::thread([this]() { threadMain(); });
+    std::thread mThread;
     std::condition_variable_any mCondition;
     std::mutex mMutex;
     std::queue<QueueEntry> mQueue GUARDED_BY(mMutex);