simpleperf: fix calculating cpu frequency.

When calculating cpu frequency and other frequencies, use time
running on cpu instead of total passed time. This is to be
consistent with linux-tools-perf.

This is reported in https://github.com/android-ndk/ndk/issues/644.

Bug: none
Test: run simpleperf_unit_test.
Change-Id: Ic385939c189fb86c6b3c648d2c0a4e9636c13c93
4 files changed