simpleperf debug-unwind: reduce time building thread maps.

When unwinding all samples in a recording file, it takes a lot of
time creating updated maps at each sample. So switch to
modify mmap records when reading them.

Bug: none
Test: run simpleperf_unit_test
Change-Id: Iad1aa229acf0fc62eef192e5e3c1d4094bb98540
2 files changed