libgui: fix an EGLImage leak

This moves the call to ConsumerBase::abandon from the ConsumerBase dtor to
ConsumerBase::onLastStrongRef.  The abandon call relies on virtual methods to
perform the clean-up, so calling it from the ConsumerBase dtor after the
derived classes dtors ran was skipping some of the clean-up.  The
onLastStrongRef method should get called just before the most derived class's
dtor gets called.

Bug: 8349135
Change-Id: I836946826927cc1ed69c049049f525f92b17a269