[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