Camera3StreamSplitter: Split out a refactorable file

Doing this so that I can easily flag guard the splitter and A/B test the
different implementations.

BYPASS_IGBP_IGBC_API_REASON=warren buffers

Bug: 340933206
Flag: com.android.graphics.libgui.flags.wb_stream_splitter
Test: atest cameraservice_test
Change-Id: Ie6769b1b2fba6d63fb054c815d19e2d78ea853e6