DO NOT MERGE SoundPool: add lock for findSample access from SoundPoolThread.
Sample decoding still occurs in SoundPoolThread without holding the SoundPool
lock.
Bug: 25781119
Change-Id: I11fde005aa9cf5438e0390a0d2dfe0ec1dd282e8
(cherry picked from commit 0275a982)
(cherry picked from commit 0a887bcd)