audio policy: protect against null input device selection

Made two improvements in audio policy engine to avoid problems when
no suitable input device can be found for a given use case:
1) make sure getDeviceForInputSource() returns a valid device for
VOICE_COMMUNICATION use case.
2) in the general case, do not crash if getDeviceForInputSource()
returns a null device.

Bug: 274001478
Test: make
Change-Id: Ib7ad122b32f54cc429f0b4ab3ff58d4ff04ea09b