CachedSet::dump: show the PixelFormat

This helps verify we're using the intended PixelFormat for screen decor
layers, and is also more generally useful. While I'm touching this code,
generally make the single-layer and multi-layer more consistent by
printing the buffer pointer in both cases. Also, only print the buffer
pointer if it's non-null.

Bug: 193170859
Test: manual

Change-Id: I71e7a8eecefa8a757bf72c487bc44900e8e0a846