Camera3StreamSplitter: Add unit tests.

I'm going to be refactoring this as part of go/warren-buffers. Having
some direct tests is helpful to make sure the transition is clean.

BYPASS_IGBP_IGBC_API_REASON=warren buffers

Bug: 340933206
Flag: EXEMPT test
Test: new test
Change-Id: Ibf68aa167e1763c96288c17780717a2909def0dc