simpleperf: add tests for scripts.

Also adjust scripts based on test results:
1. Use `pidof` in app_profiler.py.
2. Improve the way finding source files in annotate.py.
3. Change report.py to be a python wrapper of simpleperf report command,
   so users don't need to find different simpleperf binaries for different platforms.
4. Change README.md accordingly.

Bug: http://b/63006886
Test: run test.py on all platforms.

Change-Id: I00b098a1c72824226e509d7b1e5405c7cc43b856
24 files changed