Add a name to the previously-unnamed thread created by SoundPoolThread.

SoundPool itself also creates a thread, which was called "SoundPoolThread", unrelated to the SoundPoolThread class.  The SoundPoolThread class then created an unnamed thread, which showed up as "android:unnamed_thread".  That's confusing, so this change renames the SoundPool thread to "SoundPool" and then names the SoundPoolThread thread as "SoundPoolThread".  Say that ten times fast :)

Change-Id: I67b7e644a30c94b6eda44bf970764a52a1c2958b