Only support buffers with layer count == 1

Multi-layered buffers are not something that makes sense for
SurfaceFlinger to ingest. Instead of weird rendering glitches
downstream, reject those proactively to inform clients of
their mistake

Bug: 392541013
Flag: EXEMPT bug fix
Test: build & boot, existing usages work as expected
Change-Id: I7f0581b8a76accf337b9dc3212de551832374e6f