audioserver: Use '_exit' instead of 'exit' in HalDeathHandler

Using 'exit' from an RPC threadpool thread is not safe, as 'exit'
runs atexit handlers that destroy global objects. This can interfere
with code still running on other threads.

'_exit' does not run atexit handlers, just terminates the process.

Bug: 116665972
Test: kill android.hardware.audio@2.0-service, check logcat
Change-Id: I5391a659e359e0ca5bba580f1c51dea5df3ea562