Refactor some code in surfaceflinger in preparation of upcoming changes

the new TextureMagager class now handle texture creation and upload
as well as EGL image creation and binding to GraphicBuffers. This is
used indirectly by Layer and directly by LayerBuffer

the new BufferManager class handles the set of buffers used for a
Layer (Surface), it abstracts how many buffer there is as well as
the use of EGLimage vs. regular texture ops (glTexImage2D).

Change-Id: I2da1ddcf27758e6731400f6cc4e20bef35c0a39a