simpleperf: export symbol_len in report lib interface.

It helps to set the range of a function when annotating source code
and disassemble code.

Also add comments in simpleperf_report_lib.py to make the meaning
of the structures clear.

Bug: http://b/66914187
Test: run test.py TestReportLib.
Change-Id: I1be996df8062ddb6759e9f98e0400f3a4df7efb6
3 files changed