binderSafeInterfaceTest: avoid zombie

On 64-bit cuttlefish (and some other reported devices), this test was
failing to shut down properly (became a zombie process). In order to fix
this, this now forks a server much earlier, to avoid child processes
holding any resources.

Also, as an effect:
- no longer starting threadpool (only 1 thread is needed to process
test)
- using prctl to exit child process even in error cases

Bug: 174621701
Test: atest binderSafeInterfaceTest
 (on 64-bit cuttlefish, no longer hangs)
Change-Id: I326c42d0664e18c478efce83f935841e6dbfa8f3