aaudio: unlock when joining the timestamp thread

This will prevent a deadlock in case the timestamp
thread tries to acquire the same lock.

Bug: 182852602
Bug: 153358911
Test: plug and unplug headphones while playing
Change-Id: I625d191906c7e280f3a223f476716ef17b9098ea
Merged-In: I625d191906c7e280f3a223f476716ef17b9098ea
(cherry picked from commit 5f6fda778bf35be4cd67363ca0fe40cf710364c3)
1 file changed