libaudiohal: Use 'exit' in HalDeathHandler instead of fatal log

Fatal log is registered as a process crash. Since these crash
reports do not bring in any useful info, it's better to just
exit the process instead.

Bug: 69379459
Test: kill audio HAL, check logcat
Change-Id: I0d83d496998248faf3669a1fe8c31944b839bb3e