Rework logcat handling

Only extract logs from dex2oat instances that were started by the
correct runtime. This is accomplished by getting pid of runtime
and then filtering logcat output based on dex2oat 'parent PID ='
messages.

Test: bisection_search.py -cp classes.dex --class Test
  --expected-output out --device

Change-Id: Ib1a5da6fac1560c57af8245e3a921d10bf464f2a