simpleperf: fix --app option for multiprocess apps.

1. Search all processes in an app when using --app option in record command.
2. Fix searching one app process when using -p option in app_profiler.py.
3. Add unittest for profiling multiprocess apps.

Bug: http://b/65025325
Test: run test.py

Change-Id: Iba2e97c2174815d0236636e3cf15b1fc17a5d838
13 files changed