simpleperf: add interval to stat print

Add a stat interval printing

Bug: http://b/31806433
Test: simpleperf_unit_test --gtest_filter=stat_cmd.interval_option
Change-Id: Ia2881d990036cc099baccb8a2d8cd3828877a910
(cherry picked from commit e6d8a52400fe710b743ce57b89bd015ab219434b)
2 files changed