simpleperf: improve script interface.

1. In ReportLib, return structure instead of pointer to structure.
So users can use 'sample' instead of 'sample[0]', as in report_sample.py.

2. For python3, convert char pointer to str type. Because str is more
convenient to use.

Bug: http://b/32834638
Test: run simpleperf_report_lib.py and report_sample.py.
Change-Id: Iee8c3570e82104c5dd02f3f657ac143d1fdc2233
2 files changed