libgui: Add a flag for the CameraStreamSplitter refactor

We'll use this flag to pick the current (deprecated) or new
(IGBP/IGBC-free) versions of this class.

This is a libcameraservice class, but we'll put the flag here so it's
close to th other wb flags.

See go/warren-buffers for more information.

Bug: 340933206
Flag: EXEMPT adding flag
Test: n/a
Change-Id: I40f4ca09084620ba92efcac2ed6cf1be36b2dd95