Recreate EGLImage for previously used slots

SurfaceTexture would only create an EGLImage for a buffer slot when
BufferQueue returns a GraphicBuffer, i.e. either the slot was acquired
for the first time ever, or the buffer for the slot was reallocated.
But the EGLImage may also need to be re-created for a
previously-acquired buffer if the slot's EGLImage was destroyed during
detachFromContext(); in this case BufferQueue won't return a
GraphicBuffer since SurfaceTexture already has a reference to the
correct buffer.

Bug: 6461693
Change-Id: Ib95d0d757192efe336c5fda0866f857481a6617d