configstore: add new configuration item

This change adds to configstore@1.0 HAL a new configuration item
for surfaceflinger; useTripleFramebuffer()

In the default implementation, useTripleFramebuffer() will return true
only if the build flag NUM_FRAMEBUFFER_SURFACE_BUFFERS is defined as 3.

Bug: 34723491
Test: build & run. log message "SurfaceFlinger FrameBuffer: ?"
      should be seen.

Change-Id: I7ab9f17f43d68ee7b37b5db8c24b542db4d1c216
Merged-In: Iac97e25cd69717c73a135f6b2a44538124544987