speech: Fix RemoteSpeechReco race condition

stopListening schedules a Runnable referring to mDelegatingListener.
mDelegatingListener can be set to null before the Runnable runs, causing
NullPointerException when it does run.

This CL defines a local alias for the current mDelegatingListener reference,
and uses this new alias in the Runnable closure.

After this CL, even if mDelegatingListener changes between stopListening
and run(), run() will always use the DelegatingListener
mDelegatingListener was pointing to at schedule-time rather than
run-time.

Bug: 193046622
Test: atest CtsVoiceRecognitionTestCases
Change-Id: I98b4fabf074873ce17e4046e243fe22f9e541f91