simpleperf: improve warning msg.

simpleperf: improve warning msg.

The warning of missing symbol for each lib is shown twice,
once in recording, and once in reporting. This is redundant and
maybe misleading. Because when reporting we actually don't know
if we have collected enough symbols in perf.data. So move warning
of missing symbols to debug info if we have symbols in perf.data.

Use customized StderrLogger, because simpleperf doesn't need
time,pid,tid information in log.

Bug: http://b/29574526
Test: run simpleperf manually and run simpleperf_unit_test.
Change-Id: I9baf6d8fdcd63907681f2daa45b8fad6bf7e2516