simpleperf: support reporting trace offcpu data in simpleperf_report_lib.

Export python interface for reading meta info.
Change sample.period to sample time difference when trace_offcpu is true.
Add unit tests.

Bug: http://b/37572306
Test: run python test.py.
Change-Id: Ic144314184d115cc55630d3c69b66a0d8594721a
5 files changed