Surface: improve buffer look up by slot id

Before, we were comparing buffers only by handle, which may not actually
be consistent for the same buffers since the fds can be duped.

Now, we use both the handle, sp, and the buffer id for the check.

Additionally, we are now collecting all the slots being used and
reporting them as an error, as multiple slots per buffer is a bad state.

Bug: 413059222
Flag: EXEMPT small fix
Test: surface tests
Change-Id: I68fbe0ed41b7f4222de5819fad81da56bf410c8b