core: jni: fix memory leak in SoundTrigger JNI

jExtras object created in android_hardware_SoundTrigger
JNISoundTriggerCallback::onRecognitionEvent()
is not deleted causing a memory which depletes JNI
JNI local storage during back-to-back SoundTrigger
detections.
Delete jExtra after storing in return object.

Change-Id: Ib7048917b7538a27d4519504b30252879acc5447
CRs-fixed: 733969