Track the app forked processes by using the accounting info

Previously we always scan all of the processes we found and check
their parent processes, this is pretty expensive. Now we extract
the app forked processes from accounting info of app process list.

Bug: 170154111
Test: atest AppChildProcessTest
Test: atest CtsAppTestCases:ActivityManagerTest
Change-Id: Ia2210dcdada9f221d632bcfe8d4dc248eeeb8c9f