simpleperf: allow recording events in different speed.

simpleperf: allow recording events in different speed.

Currently record command interface only allows one sample freq or sample period
for all events. This is not convenient when recording both non tracepoint events
and tracepoint events. This CL allows setting different sample speed for
different events. For example, for "-f 1000 -e cpu-cycles -c 1 sched:sched_switch",
"-f 1000" applies to cpu-cycles, and "-c 1" applies to sched:sched_switch.

It also fixes a bug about trace-offcpu: if "-f 1000 --trace-offcpu" is used,
the sched:sched_switch is be samples with "-f 1000". But we want to sample it
with "-c 1".

Also change the order of options in the help msg of record cmd to make it more
readable. Remove -F option. Because adding it seems not useful.

Bug: http://b/37572306
Test: run simpleperf_unit_test.

Change-Id: Ifdbd27c8f9fec49aade0e0e6ce624d8114042020