audio: Change the default init value for StreamDescriptor

Currently, audio doesn't play on Brillo because the policy manager's
default value for all streams is set to 0. In Android, this value is
changed by calls from AudioService.java. If we set the default value to
1 in Brillo, then even if there are no calls, we will have audio.

BUG=none
TEST=manual

Change-Id: I1418c9ecfb4f1c81c9c0946eee11deed3c65b358