simpleperf: Fix reading old branch list files

We have moved to a new branch list file format (in aosp/3341393).
But we want to keep the ability of reading old branch list file
format for some time. This patch fixes the support, and adds a
test for it.

Bug: 345410289
Test: run simpleperf_unit_test
Change-Id: If4c190e61347be8cebea84fa28bc83a599dc41cd
4 files changed