simpleperf: support event group.

Kernel supports monitoring several events as a group, so they are
scheduled on and out at the same time. Add --group option to
stat command and record command.
Adjust the method to calculate miss rate in stat command: limit
the matched events in the same group or with scale == 1.0.

Bug: 29213742

Change-Id: I899aba207f1e3357307541e81f97526f5a2913c3
11 files changed