updates to FrameStatsObserver API

- Rename to FrameMetrics to avoid collision with existing
  android.view.FrameStats class
- Make FrameMetricsObserver implementation detail,
  exposing FrameMetricsListener interface as public API
  and wrapping in FrameStatsObserver to maintain state
- Remove dropped frame count call, in favor of passing as
  parameter to callback method.
- Move away from raw timestamp access in favor of Metric IDs
  which represent higher-level, more stable stages in a frame
  lifecycle and match the categories exposed in the onscreen
  bars.
- Support many-to-many Window<->FrameMetricsListener relationship

Change-Id: I00e741d664d4c868b1b6d0131a23f8316bd8c5c2