simpleperf: fix scripts.

Fixed scripts based on test results on different platforms:

1. Decrease sample frequency from 4000 to 1000 when using
-g option. Because using -f 4000 sometimes makes a high
loss rate >= 50%.

2. Use kill when am force-stop can't kill the app process,
this happens when testing on Android N.

3. In app_profiler.py, only download native libs when
profiling an app.

4. Fix some small errors.

Bug: http://b/32834638
Test: run python test.py on linux/windows/darwin
Test: with Android N/O devices.
Change-Id: If8f7ab0195e87014c3ad4ab0d007d0eac8b6f528
4 files changed