audio flinger: force audio path start from normal mixer

The audio HAL wakes up and configures the audio path when receiving
the first write() in standby state. This causes a certain amount of
process to take place in the mixer threads which is problematic for
fast mixer running at FIFO priority.

We now force a fake write() of 0 bytes from normal mixer to trigger
the audio path configuration before starting the fast mixer.

Bug: 23791972.

Change-Id: I54311b337fda956444846f5d2f53a3263d54e04b