simpleperf: accumulate event counts in SampleEntry.

Add new fields in SampleEntry to store event counts. Add code
in ReportCmdSampleTreeBuilder to parse event counts from SampleRecord,
and merge event counts in MergeSample().

Bug: 211001574
Test: run simpleperf_unit_test
Change-Id: Ie40ec955954a749a0243f9e01046ae122d444d16
1 file changed