Bluetooth: Hedge against the Null global reference access

Whenever there is an error at upper layer it cleans up JNI
interface as part of cleanup and global reference also get
dereferenced but lower layer HAL is not aware in case of upper
layer and sends the jni callback where it crashes.protecting
all callbacks since expereincing similar issues at different
callbacks.

Change-Id: Idab75ca537e02f976e6892d7e03aa70fb7e43868
CRs-fixed: 710760