Add mutex locking to protect OpenSL calls

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Add additional race protection

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Add mutex protection for OpenSL buffer queue

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Add mutex protection for JNI calls

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Minor refactoring in playlist button listeners

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Update changelog

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>