SF Timestats: add new metrics

Add the new metrics below:
1) dropped buffers per layer
2) post to acquire histogram

Stop tracking the start and end time for each layer since that doesn't
give any meaningful information. The total/dropped frame per layer is
good enough for now.

Test: dumpsys SurfaceFlinger --timestats <see go/sf-timestats for args>
Bug: b/110954467
Change-Id: I1c8a3a6cc76b7ec4a2e9ab86d9237af95fe9d1e1