simpleperf: add --brief-callgraph option for report cmd.

Remove duplicated callgraphs (which appears as a sub graph in
another place) in --brief-callgraph mode.
Accept brief callgraph in report.py.
Add unit test for --brief-callgraph option.

Bug: http://b/37444055
Test: run simpleperf_unit_test.
Test: manually run report.py.
Change-Id: I41977762552ec66807f394558352da6cbefaec2e
7 files changed