simpleperf: support libraries without build ids when recording.

simpleperf: support libraries without build ids when recording.

Simpleperf can download unstripped libraries on device and use them
for unwinding during recording, through `app_profiler.py -lib` and
`simpleperf record --symfs` options. But it doesn't support libraries
without build ids.

To support them:
1. In app_profiler.py, download libraries without build ids in native
   lib dir.
2. In DebugElfFileFinder in dso.cpp, check path with the same basename
   in symfs_dir.

Also add document for downloading unstripped libraries on device.

Bug: none
Test: run simpleperf_unit_test.
Test: run test.py.
Change-Id: I5d9015e683f2ecb992d425a42f1f7303307b2cea