configstore: removing BoardConfig variables (NUM_FRAMEBUFFER_SURFACE_BUFFERS)

This change removes the use of NUM_FRAMEBUFFER_SURFACE_BUFFERS from
SurfaceFlinger.

Instead of using it, the implementation is changed so that it retrieves
corresponding configuration value (useTripleFramebuffer) from
configstore.

Design doc: go/design-confighal
Bug: 34723491
Test: build, run, log message "SurfaceFlinger FrameBuffer: ?" should be seen.

Change-Id: If6f6d13738d894b4880f42576f4ce8220b4a8bef