simpleperf: allow --duration option when executing child command.

Previously --duration option can't be used while executing child
command, mainly because we use `sleep XX` child command to implement
--duration option. By using libevent, the limit doesn't exist.

Bug: http://b/30405638
Change-Id: Iea1953ecf3aca48e75fb74f048ed04ad5b168630
Test: run simpleperf_unit_test.
4 files changed