libusbhost: The client is now responsible for creating the thread that monitors the bus

This is to allow using a thread that is capable of calling through JNI to Java code
to report USB device attached/removed events.

Change-Id: Ia58592607a2c1f4357b31072044f5db5617d7f5b
Signed-off-by: Mike Lockwood <lockwood@android.com>