StreamSplitter: Ignore already-owned buffers when attaching

This is the old behavior that got dropped when transitioning to Surfaces
from IGBPs. Adds a new test to verify.

The flag roll out caught the bug in MultiViewTest.testSharedSurfaces.
See ag/31219568.

BYPASS_IGBP_IGBC_API_REASON=warren buffers

Bug: 340933206
Flag: com.android.graphics.libgui.flags.wb_stream_splitter
Test: MultiViewTest.testSharedSurfaces, new test too

Change-Id: Ieb5e809f25d22820477b5e2724793c992e3d5ef0