Add a list to track unfinished tasks and dumpStackTraces

Previously exception message only prints unstarted tasks in the thread
pool. We also want to know the started but unfinished tasks. Add an
array list mPendingTask to track all unfinished tasks.

Also dumpStackTraces() so we can tell why the task in the threadpool
failed to finish on time.

This will help us to find root cause of IllegalStateException in
SystemServerInitThreadPool.shutdown().

Bug: 128928718
Test: Reboot device, adb logcat | grep SystemServerInitThreadPool. Force
a task to sleep for 30 seconds, observe IllegalStateException with
unfinished task printed and stack traces is dumped in a ANR file.

Change-Id: I98ba86e58ae1f2e1ab2b3f8ea3724c6b70a796eb