AudioFlinger playback thread CPU measurement in Hz

Log statistics on CPU usage in Hz in addition to wall clock time

Use CPU statistics for all playback threads, not just MIXER
(but they are disabled by default by a compile-time debug macro).

ThreadCpuUsage library:
 - Move statistics out of the library and leave that up to the caller
 - Add API to determine a CPU's frequency

Change-Id: Ia1011123146e641fcf210ef26e78ae2b4d3b64ad