audio: Add CALL_SCREEN audio mode.

Add new audio mode AudioMode.CALL_SCREEN allowing call screening
to take place while other audio use cases are still active.

Also add audio policy configuration attribute indicating if the
vendor implementation supports this audio mode.

Bug: 140384450
Test: make
Change-Id: I2714a9949f2c45b1f8e5a5c40368a6152bd91572