Add hotword AudioService null check

If an application tries to start recording with the HOTWORD input before
AudioService is ready, the interface is not available to call
handleHotwordInput on. Check for null and only log if AudioService is
not available.

Change-Id: I2e693c5aed7cdce6299d8ff16341c1af8dd5b54f
Issue-Id: FOR-257
(cherry picked from commit 801a2348)