HwBinder: Clean up code, add logging

Used ScopedUtfChars to handle lifetime, and correctly throw NPE. It
is illegal to call JNI String functions with a null argument.

Output the service name when starting a thread pool to aid debugging.

Bug: 68217725
Test: m
Test: Device boots
Change-Id: Ie40d3e77a63918330ef1a8a9e0c294f2c451a52a